Every 
ing group has a goddamn Assassin Rogue. Every ing one. I'm tired of it. I want to build a Thief. But let's be honest: Thief kinda sucks in comparison. It has a great capstone, but Assassin Rogues get an equally impressive one. So it, if we're gonna be good Thieves, we gotta throw in a splash of something else to capitalize on our strengths. We need to multiclass to be an optimal choice.
We'll never do more damage than Assassins, so let's not even build that way. What are a Thief's strengths? Being fast and being sneaky. Specifically, they can use their hands as bonus action, climb faster than everyone else, sneak with Advantage, use magic items that aren't meant for them, and get an extra turn on the first round of combat. So let's focus on these and optimize these.
Fast Hands, 3. The best use of this I can think of is to apply a poison to your weapons/ammo and attack on the same turn. That's actually pretty useful! Some poisons do a shitload of damage, so you might even out-damage an Assassin Rogue. Doubtful though, since it's really hard to get your hands on high damage poisons. Since our damage isn't as reliable as Assassins, I'd rather stick to Sleep, Blind, and Paralyze type poisons to control the fight. Any class with Extra Attack can synergize well by applying these control poisons to multiple foes faster, making it more efficient.
Fast Hands would work well with Use Magic Device, to quickly activate your items before using your main action, but I can't think of any magical devices that would be useful in combat. Grab something out of your Bag of Holding maybe? That's not really taking advantage of Use Magic Device though, since Rogues can already use Bags of Holding.
Other ideas for Fast Hands: drop Ball Bearings or Caltrops between you and your foe, lay a Hunting Trap, fasten Manacles (handcuffs) on an enemy next to you, throw Holy Water at a fiend or undead, heal an ally with a Healer's Kit, drink or administer a Potion of Healing (or any potion for that matter). Fastening Manacles and then using Heat Metal would be good synergy. Druids and Bards learn Heat Metal as early as level 3. Heat Metal would also work on anyone who stepped on the metal Hunting Trap. Clerics and Paladins usually carry Holy Water for good synergy in campaigns with lots of undead. If your group needs a healer, but you don't want to take a healing class, applying Healer's Kits and Potions of Healing without sacrificing your main action would be nice. Remember, your allies only need 1HP to be 100% useful. Anyone with access to Levitate or Fly would be good for Ball Bearings/Caltrops throwers.
One thing that caught my eye was Oil. You can throw Oil on an enemy and it'll take an extra 5 fire damage if you can use your main action to attack with fire. One way to do this is to attack with a lit Torch, but that only does 1 damage (+5). If we can multiclass to cast a fire cantrip/spell then we can capitalize on this. You could just take the Magic Initiate feat too though, so I don't know... Yes, Arcane Tricksters can cast these, but they don't have Fast Hands to throw the Oil as a bonus action, so that gives us a small (non-scaling) advantage.
Fast Hands is also good for disarming traps. You can take the Dodge action to give you Advantage on DEX saving throws (in case you trigger the trap), then take the bonus action to attempt to disarm the trap. Of course, you need to find the traps first. Clerics, Rangers, and Wizards provide good multiclass synergy because they learn the spell Find Traps! You already get Investigation as a Rogue, as well as Expertise, so we don't need Bard for that. Bard can steal Find Traps as well as Pass Without Trace though, so it's worth considering.
Second-Story Work, 3. You can climb walls faster than anyone... Okay, let's think outside the box. What if we...made more walls to climb? Wall of Stone is a 5th level spell for Druids, Sorcerers, and Wizards. Bards can steal this too. It's actually a pretty good spell. You can enclose enemies, divide enemies, or protect your squishy allies. Unfortunately, you need 9 levels of a full caster class to get this. Fortunately, you get the wall-climbing bonus at level 3, meaning Thief is an interesting multiclass dip for a caster that wants to use this spell often. With access to other spells like Passwall, Stone Shape, and Investiture in Stone, I can see someone building a character around this. Passwall is particularly nice, because you can use it for trenching yourself and your foes at the same time. You can climb out with ease while they struggle.
Another tactic is to climb the already-existing walls of whatever room you're in to get out of reach of enemies. It means you'd have to rely on Ranged weapons to do attacks, but that's alright. You can't hide though, so no Sneak Attack... If you're going to climb walls and make Ranged attacks without relying on Sneak Attack for damage, Ranger and Fighter dips come to mind. But again, we're never going to out-damage Assassin Rogues, so why are we doing this? It's not ideal to build around this tactic.
Supreme Sneak, 9. This is what makes Thief great. Advantage on Stealth! Sneaking and scouting just became twice as easy. Other than Rogues, what other classes benefit from being Sneaky? Druids and Rangers are natural scouts. For synergy, Druids and Rangers have Pass Without Trace at level 3 (Trickster Clerics get this at 3 too, and Bards can steal it at 6). Expertise Stealth +10, Pass Without Trace +10, and Advantage... To fail at the Stealth check, you'd have to roll 2 Nat 1s in a row (1 in 400 chance). Advantage on Stealth checks is good for every class though. I can see Heavy Armor-wearing classes like Clerics, Fighters, and Paladins who normally get Disadvantage on Stealth synergizing well with Thief. For example, a Thief main can dip into Fighter for Action Surge and wear Heavy Armor now without taking a risk, since the Advantage and Disadvantage cancel each other out.
Use Magic Device, 13. I'm having trouble with this one, mostly because I haven't memorized the items in the DMG. Also, DMs often make up unique magical items and devices for each campaign. Wizards can sometimes acquire powerful magic items from their university, so there's that. Even 1 level of Wizard will get you "in" to the university, and you can pick a few locks to gain access to rooms you shouldn't be in. After all, you are a Thief.
Thief's Reflexes, 17. Okay, this is pretty high level, so I doubt most players well get here. But I do have a few ideas for this one. First of all, PCs always use the first round of combat for buffing and preparing themselves for the long fight. So, a good way to take advantage of this double turn on round one is to buff everyone. You can only multiclass 3 levels, for access to 2nd level spells. That means no Haste or Stoneskin. You're limited to spells like Bless, Faerie Fire, Invisibility, and Longstrider. Don't get me wrong, those are actually not half bad, but by the time you're level 18+, it's probably not going to make a big difference. Bless looks to be the best of all those, or Longstrider if you're worried about Concentration. You could get these from Magic Initiate (except Invisibility, which will only provide an ally with Advantage on their first hit anyway), so I don't know if it's worth a multiclass dip for this...
Instead of buffing allies, we can use this extra turn to debuff foes. Taking a caster with control spells would work well, but your Spell Save DC is probably gonna be crap. Spells like Blindness/Deafness, Charm Person, Command, Entangle, Hold Person, Silence, Sleep, etc are all going to fail. A better idea would be to use Manuevers from a Battle Master Fighter dip to knock a few enemies prone or disarm them right off the bat. You can get Manuevers from the Martial Adept feat though, without having to multiclass. I will say that Fighter is nice synergy for Action Surging on that first round (3 chances to prone or disarm on round 1).
Builds
Battle Master Fighters synergize great with Thief. Thief 10/Battle Master 10. Fast Hands+Extra Attack=applying control poisons to 2 enemies instead of just one. Heavy Armor+Supreme Sneak=Disadvantage on Stealth is cancelled by Advantage. Action Surge lets you Manuever for 2 disarm/prones on the first round. If you want to give up Extra Attack (I wouldn't) and go 17/3, then Action Surge+Thief's Reflexes=3 actions on the first round including Manuevers to disarm/prone. You'll never out-damage Assassins, but at least as a melee multiclass you can use Sneak Attack. It's hard to decide on the level split here, because Battle Masters give such nice benefits all the way up to 10. If you can find a good use for Use Magic Device, then 13/7 would be another good split.
Warlocks synergize great with Thief. Warlock 17/Thief 3. You're just in it for Fast Hands. Ascendant Step gives you at-will Levitate, so throw Ball Bearings and Caltropsing everywhere, every single round. Your main action is still free for casting effectively. 3 levels of Rogue gives you a lot of other nice goodies, so it's not a bad investment for an otherwise full caster. You get an extra skill, Expertise, and Cunning Action. Repelling Blast will knock enemies into your Ball Bearings and Caltrops for some fantastic synergy. You could go Pact of the Blade with Thirsting Blade to make use of Sneak Attacks and Lifedrinker for when enemies actually make it past your defenses, but any pact would be strong even without using Sneak Attack. Pact of the Chain gives you a demon Abu, which would definitely come in handy as an infiltrator.
Trickster Clerics synergize well with Thief. Thief 17/Trickster 3. Fast Hands+Find Traps=best trap finder/disarmer combo in the game. Like I said before, Fast Hands lets you use Dodge as your main action for the DEX save bonus before using your bonus action to attempt the disarm. Pass Without Trace+Supreme Sneak=nearly impossible to fail Stealth checks. Thief's Reflexes+Bless=free party buff on round one. Low spell slots is the only thing that really holds this build back.
Lore Bards synergize well with Thief. Lore 10/Thief 10. Additional Magic Secrets gives you Find Traps and Pass Without Trace at 6, making you into a Trickster Cleric. At 10, you can pick up Wall of Stone and Passwall. You lose out on some Sneak Attack dice, but you were never the best damage dealer out there anyway. You lose things like Blind Sense, Reliable Talent (helps with Stealth), and Slippery Mind (proficiency in Wisdom saving throws is pretty good...), but pick up a lot of Bard goodies along the way like d10 Cutting Words with Font of Inspiration, Expertise and Jack of All Trades, and of course those 4 Magical Secret spells. You also have Heat Metal+Fast Hands=Manacle an enemy and burn them on the same round.
Grassland Land Druids synergize okay with Thief. Thief 17/Grass 3. Wild Shape gives you great utility for infiltrating. You can be extra sneaky or turn into a rat that people will ignore even if they spot you. Grassland gives you Pass Without Trace, meaning you'll never fail your stealth checks anyway. You get refreshing spell slots, Invisibility, backup heals. It's an interesting choice, but losing out on Find Traps makes you less optimal at your role than a Trickster Cleric or Lore Bard.
Fire Casters synergize meh with Thief. Fire Dragon Sorcerer 17/Thief 3 for instance. Use bonus action each turn to throw Oil on an enemy before casting a fire spell on them. It's not much extra damage, but unless you use Quicken Spell every single round, you have nothing better to use your bonus actions on. Plus, like I said earlier, taking Rogue to 3 for an extra skill, Expertise, and Cunning Action is not a bad investment, even for a full caster. Wizards get a bunch of useful spells for an infiltrator, so if you're going to go this route, I think Wizard would be better than Sorcerer. All this trouble though for an extra 5 fire damage a round is...questionable.
Other options synergize poorly. Yeah you might have a trick or two, like Rangers getting Pass Without Trace. But come on. You might as well just go Assassin Rogue, because Thief gives you little to synergize with.
ing group has a goddamn Assassin Rogue. Every ing one. I'm tired of it. I want to build a Thief. But let's be honest: Thief kinda sucks in comparison. It has a great capstone, but Assassin Rogues get an equally impressive one. So it, if we're gonna be good Thieves, we gotta throw in a splash of something else to capitalize on our strengths. We need to multiclass to be an optimal choice.We'll never do more damage than Assassins, so let's not even build that way. What are a Thief's strengths? Being fast and being sneaky. Specifically, they can use their hands as bonus action, climb faster than everyone else, sneak with Advantage, use magic items that aren't meant for them, and get an extra turn on the first round of combat. So let's focus on these and optimize these.
Fast Hands, 3. The best use of this I can think of is to apply a poison to your weapons/ammo and attack on the same turn. That's actually pretty useful! Some poisons do a shitload of damage, so you might even out-damage an Assassin Rogue. Doubtful though, since it's really hard to get your hands on high damage poisons. Since our damage isn't as reliable as Assassins, I'd rather stick to Sleep, Blind, and Paralyze type poisons to control the fight. Any class with Extra Attack can synergize well by applying these control poisons to multiple foes faster, making it more efficient.
Fast Hands would work well with Use Magic Device, to quickly activate your items before using your main action, but I can't think of any magical devices that would be useful in combat. Grab something out of your Bag of Holding maybe? That's not really taking advantage of Use Magic Device though, since Rogues can already use Bags of Holding.
Other ideas for Fast Hands: drop Ball Bearings or Caltrops between you and your foe, lay a Hunting Trap, fasten Manacles (handcuffs) on an enemy next to you, throw Holy Water at a fiend or undead, heal an ally with a Healer's Kit, drink or administer a Potion of Healing (or any potion for that matter). Fastening Manacles and then using Heat Metal would be good synergy. Druids and Bards learn Heat Metal as early as level 3. Heat Metal would also work on anyone who stepped on the metal Hunting Trap. Clerics and Paladins usually carry Holy Water for good synergy in campaigns with lots of undead. If your group needs a healer, but you don't want to take a healing class, applying Healer's Kits and Potions of Healing without sacrificing your main action would be nice. Remember, your allies only need 1HP to be 100% useful. Anyone with access to Levitate or Fly would be good for Ball Bearings/Caltrops throwers.
One thing that caught my eye was Oil. You can throw Oil on an enemy and it'll take an extra 5 fire damage if you can use your main action to attack with fire. One way to do this is to attack with a lit Torch, but that only does 1 damage (+5). If we can multiclass to cast a fire cantrip/spell then we can capitalize on this. You could just take the Magic Initiate feat too though, so I don't know... Yes, Arcane Tricksters can cast these, but they don't have Fast Hands to throw the Oil as a bonus action, so that gives us a small (non-scaling) advantage.
Fast Hands is also good for disarming traps. You can take the Dodge action to give you Advantage on DEX saving throws (in case you trigger the trap), then take the bonus action to attempt to disarm the trap. Of course, you need to find the traps first. Clerics, Rangers, and Wizards provide good multiclass synergy because they learn the spell Find Traps! You already get Investigation as a Rogue, as well as Expertise, so we don't need Bard for that. Bard can steal Find Traps as well as Pass Without Trace though, so it's worth considering.
Second-Story Work, 3. You can climb walls faster than anyone... Okay, let's think outside the box. What if we...made more walls to climb? Wall of Stone is a 5th level spell for Druids, Sorcerers, and Wizards. Bards can steal this too. It's actually a pretty good spell. You can enclose enemies, divide enemies, or protect your squishy allies. Unfortunately, you need 9 levels of a full caster class to get this. Fortunately, you get the wall-climbing bonus at level 3, meaning Thief is an interesting multiclass dip for a caster that wants to use this spell often. With access to other spells like Passwall, Stone Shape, and Investiture in Stone, I can see someone building a character around this. Passwall is particularly nice, because you can use it for trenching yourself and your foes at the same time. You can climb out with ease while they struggle.
Another tactic is to climb the already-existing walls of whatever room you're in to get out of reach of enemies. It means you'd have to rely on Ranged weapons to do attacks, but that's alright. You can't hide though, so no Sneak Attack... If you're going to climb walls and make Ranged attacks without relying on Sneak Attack for damage, Ranger and Fighter dips come to mind. But again, we're never going to out-damage Assassin Rogues, so why are we doing this? It's not ideal to build around this tactic.
Supreme Sneak, 9. This is what makes Thief great. Advantage on Stealth! Sneaking and scouting just became twice as easy. Other than Rogues, what other classes benefit from being Sneaky? Druids and Rangers are natural scouts. For synergy, Druids and Rangers have Pass Without Trace at level 3 (Trickster Clerics get this at 3 too, and Bards can steal it at 6). Expertise Stealth +10, Pass Without Trace +10, and Advantage... To fail at the Stealth check, you'd have to roll 2 Nat 1s in a row (1 in 400 chance). Advantage on Stealth checks is good for every class though. I can see Heavy Armor-wearing classes like Clerics, Fighters, and Paladins who normally get Disadvantage on Stealth synergizing well with Thief. For example, a Thief main can dip into Fighter for Action Surge and wear Heavy Armor now without taking a risk, since the Advantage and Disadvantage cancel each other out.
Use Magic Device, 13. I'm having trouble with this one, mostly because I haven't memorized the items in the DMG. Also, DMs often make up unique magical items and devices for each campaign. Wizards can sometimes acquire powerful magic items from their university, so there's that. Even 1 level of Wizard will get you "in" to the university, and you can pick a few locks to gain access to rooms you shouldn't be in. After all, you are a Thief.
Thief's Reflexes, 17. Okay, this is pretty high level, so I doubt most players well get here. But I do have a few ideas for this one. First of all, PCs always use the first round of combat for buffing and preparing themselves for the long fight. So, a good way to take advantage of this double turn on round one is to buff everyone. You can only multiclass 3 levels, for access to 2nd level spells. That means no Haste or Stoneskin. You're limited to spells like Bless, Faerie Fire, Invisibility, and Longstrider. Don't get me wrong, those are actually not half bad, but by the time you're level 18+, it's probably not going to make a big difference. Bless looks to be the best of all those, or Longstrider if you're worried about Concentration. You could get these from Magic Initiate (except Invisibility, which will only provide an ally with Advantage on their first hit anyway), so I don't know if it's worth a multiclass dip for this...
Instead of buffing allies, we can use this extra turn to debuff foes. Taking a caster with control spells would work well, but your Spell Save DC is probably gonna be crap. Spells like Blindness/Deafness, Charm Person, Command, Entangle, Hold Person, Silence, Sleep, etc are all going to fail. A better idea would be to use Manuevers from a Battle Master Fighter dip to knock a few enemies prone or disarm them right off the bat. You can get Manuevers from the Martial Adept feat though, without having to multiclass. I will say that Fighter is nice synergy for Action Surging on that first round (3 chances to prone or disarm on round 1).
Builds
Battle Master Fighters synergize great with Thief. Thief 10/Battle Master 10. Fast Hands+Extra Attack=applying control poisons to 2 enemies instead of just one. Heavy Armor+Supreme Sneak=Disadvantage on Stealth is cancelled by Advantage. Action Surge lets you Manuever for 2 disarm/prones on the first round. If you want to give up Extra Attack (I wouldn't) and go 17/3, then Action Surge+Thief's Reflexes=3 actions on the first round including Manuevers to disarm/prone. You'll never out-damage Assassins, but at least as a melee multiclass you can use Sneak Attack. It's hard to decide on the level split here, because Battle Masters give such nice benefits all the way up to 10. If you can find a good use for Use Magic Device, then 13/7 would be another good split.
Warlocks synergize great with Thief. Warlock 17/Thief 3. You're just in it for Fast Hands. Ascendant Step gives you at-will Levitate, so throw Ball Bearings and Caltrops
ing everywhere, every single round. Your main action is still free for casting effectively. 3 levels of Rogue gives you a lot of other nice goodies, so it's not a bad investment for an otherwise full caster. You get an extra skill, Expertise, and Cunning Action. Repelling Blast will knock enemies into your Ball Bearings and Caltrops for some fantastic synergy. You could go Pact of the Blade with Thirsting Blade to make use of Sneak Attacks and Lifedrinker for when enemies actually make it past your defenses, but any pact would be strong even without using Sneak Attack. Pact of the Chain gives you a demon Abu, which would definitely come in handy as an infiltrator.Trickster Clerics synergize well with Thief. Thief 17/Trickster 3. Fast Hands+Find Traps=best trap finder/disarmer combo in the game. Like I said before, Fast Hands lets you use Dodge as your main action for the DEX save bonus before using your bonus action to attempt the disarm. Pass Without Trace+Supreme Sneak=nearly impossible to fail Stealth checks. Thief's Reflexes+Bless=free party buff on round one. Low spell slots is the only thing that really holds this build back.
Lore Bards synergize well with Thief. Lore 10/Thief 10. Additional Magic Secrets gives you Find Traps and Pass Without Trace at 6, making you into a Trickster Cleric. At 10, you can pick up Wall of Stone and Passwall. You lose out on some Sneak Attack dice, but you were never the best damage dealer out there anyway. You lose things like Blind Sense, Reliable Talent (helps with Stealth), and Slippery Mind (proficiency in Wisdom saving throws is pretty good...), but pick up a lot of Bard goodies along the way like d10 Cutting Words with Font of Inspiration, Expertise and Jack of All Trades, and of course those 4 Magical Secret spells. You also have Heat Metal+Fast Hands=Manacle an enemy and burn them on the same round.
Grassland Land Druids synergize okay with Thief. Thief 17/Grass 3. Wild Shape gives you great utility for infiltrating. You can be extra sneaky or turn into a rat that people will ignore even if they spot you. Grassland gives you Pass Without Trace, meaning you'll never fail your stealth checks anyway. You get refreshing spell slots, Invisibility, backup heals. It's an interesting choice, but losing out on Find Traps makes you less optimal at your role than a Trickster Cleric or Lore Bard.
Fire Casters synergize meh with Thief. Fire Dragon Sorcerer 17/Thief 3 for instance. Use bonus action each turn to throw Oil on an enemy before casting a fire spell on them. It's not much extra damage, but unless you use Quicken Spell every single round, you have nothing better to use your bonus actions on. Plus, like I said earlier, taking Rogue to 3 for an extra skill, Expertise, and Cunning Action is not a bad investment, even for a full caster. Wizards get a bunch of useful spells for an infiltrator, so if you're going to go this route, I think Wizard would be better than Sorcerer. All this trouble though for an extra 5 fire damage a round is...questionable.
Other options synergize poorly. Yeah you might have a trick or two, like Rangers getting Pass Without Trace. But come on. You might as well just go Assassin Rogue, because Thief gives you little to synergize with.
