This thread definitely needs more Bard.
As pointed out in the January unearthed arcana bards are often assassins hanging out in plain sight.
In fact I would say thematically Valor Bard 17/Rogue 3 is one of the most thematically flavorful characters in the game. They show up, play their gig, get out a town. Though that mostly works as human, half-elf, or elf since they can blend easier in most worlds.
I'll let others calculate the nova damage but it can get pretty insane if you drop an 8th level spell slot -9th is usually better off for the powerful 9th lvl spells- on Heat Blade. The damage can get pretty crazy and you become the equivalent of Trunks slicing Frieza apart like cheddar. The damage on one's own is great but it get's crazier when you consider: 14th lvl magical secrets: Simulacrum. This either grants extra damage or creates the perfect alibi. This is really effective with a dex based bard who has the Alert feat, with jack of all trades and possibily bardic inspiration added is likely to have the initiative after the surprise round which would then grant advantage on an attack or just allow a nice quick teleport spell to flee the scene. I mean if they really have great timing they could even have a 6th or 8th level spiritual weapons (via magic secrets) up and ready to add to the attack. A Bard 17/Rogue 3 may be the most dangerous single individual in the game. It's like being a demolition expert. I'll talk more about that on the Bard thread though, instead I'll show a few Bard builds I think really work with Rogue 3/Fighter 2.
Valor Bard 15/Rogue 3/Fighter 2
As far as I can figure THE top archer in the game. Take Swift Quiver w/10th lvl Magical Secrets, get archery fighting style which then allows you to use the Sharpshooter feat on the ambush and you can rack up some damage when Simulacrum comes online. Unfortunately this build only REALLY works with Variant Human since that allows you to start with either Sharpshooter or Alert. Like I said getting the initiative on the next attack is pretty amazing, it's like a natural action surge. Oh yeah, did I forget to mention an 8th level lightning arrow ready to fly? That's also possible crazy damage, I think I saw Mellored calculate it at 270+ damage
Barbarian 9/Lore Bard 6/Rogue 3/Fighter 2
This is kinda the half-orc special but the key will be to get haste w/6th lvl Magical Secrets and if you use a maul or battle axe It's some pretty solid damage. Unfortunately only 3 ASI's for strength but still, probably a playable and fun character, a one orc or even a one man wrecking crew. I dunno if this could run as dex based but it might be worth looking at for initiative
On a side note I'd like to see a barbarian totem that fuses with the assassin, path of the Viper maybe? you could be the D&D equivalent of a hell's angel hitman.
Eldritch Knight 11/Lore Bard 6/Rogue 3
3 attacks, action surge and you'll be a 9th or 10th level caster which will allow heat blade or haste. Conversely you could go more Bard with something like EK 7/LB 10/Rogue 3 for a more powerful caster who will have 4 magical secrets but not as many attacks or ASI's.
Another one I didn't really calculate is around a paladin since that allows for smiting using bard spell slots but I'm sure something that gets to Lore Bard 6 and haste is pretty damn good.
Another great benefit is the added expertise, which I think adds alot of flavor for an Assassin.
One question though, if you roll a 20 on an auto-crit do you deal an extra die of damage? Couldn't find the answer