Contingency is an amazing spell... if you use it correctly. You have loads of great options and the best thing is that it lasts for quite a few days, making it easy on the spell slots. As a Bladesinger, you're incentivized to use the spell in ways that don't occur to most wizards, such as for Haste or Blink. The biggest thing that should be on your mind should be whether you're using Contingency for offense or using it for defense. Generally, using it for defense is stronger but speed is everything to a wizard gish. So if the idea of being immune to at least one attack doesn't float your boat, you can still use it to get a jump on the damage race.
Note that you can also use Contingency to fast-cast buffs in the middle of an extended workday. This generally isn't the best use for the spell (except for Otiluke's Resilient Sphere, see below) but when you're at very high level and you have 4 to 6 6th+ level spell slots to play with it's worth considering. Just another thing to consider if you're, say, thinking of taking two levels of Fighter for Action Surge.
Bigby's Hand: Like telekinesis, it can be used to fast-casting an offensive spell. Use the initial hand to, again, push you out of the way of an oncoming attack. Once you do so, you can use the hand on different rounds to do something else. It's safer than telekinesis since it won't restrain you if it works. Another way it's better than Telekinesis: it will only require a bonus action to control on subsequent rounds. Unless you really see yourself wanting to use telekinesis for something, this is a better choice.
Blink: See the notes on Mirror Image. Note that while Blink provides a perfect defense (unlike Mirror Image) it's not reactive. But it's not concentration-duration, so that's nice.
Blur: There's pretty much no reason to use this spell over Greater Invisibility. A situation that won't work for Greater Invis won't work for Blur. Maybe if you're using Contingency to fast-buff in the middle of a work day and want to conserve 4th-level spell slots.
Dimension Door: Very versatile. The obvious use is to dodge out of the way of an enemy's attack, but it can also be used in a pinch for extra movement if things go south such as a surprise near-TPK or being trapped in rubble or an enemy putting up a Wall of Force. Note that a strict wording of the spell makes it so that even though Contingency doesn't normally target additional people, the effect of taking someone along with you happens after the spell goes off. If your DM agrees to that interpretation, you can use Dimension Door to cover someone else's butt, too. Like, say, dodging dragon's breath. Sky blue if your DM will let you use the spell to dodge being affected with targeted effects and/or save another party member's bacon.
Dispel Magic: If you're fighting a lot of spellcasters who do evil things like trapping you in stone and dropping Contagions on you, you can use it as a backup when Counterspell doesn't get you the goods.
Fire Shield: Not the best choice in the world, as you shouldn't be getting hit that often in the first place, but it is concentration-free. Use it for concentration-free defense when you know you're going to fight dragons (freeing up your reaction for Shield and Song of Defense), though the duration on the base spell is long enough that you probably don't need to do this.
Gaseous Form: While it comes with a load of defensive benefits (advantage on three saves, gets you out of bindings, some small flight, resistance to nonmagical damage) the two big problems with this spell are its concentration duration and its inability to let you attack. The thing that saves this spell from complete uselessness as far as contingency is concerned is its concentration duration, meaning that you can drop concentration as soon as your turn comes.
Greater Invis: Great choice. Gives you loads of defense, especially if you stick to ranged attacks, and gives you advantage to boot. Depending on how your DM interprets things, this could even prevent you from being targeted by certain spells. The only thing is that it requires concentration, otherwise it'd be gold.
Haste: Another great choice. Have it pop up when you're barely hit for a free miss, or even a Dex save if you're feeling really lucky. It's also a prime target to use for fast-buffing. Sky Blue, maybe even gold, if you're a Bladesinger who likes to use two-weapon fighting.
Melf's Minute Meteors: If you upcast this spell to level 5, you get 20d6 worth of meteors spread out over a minimum of 5 rounds, 4d6 a round. Normally wouldn't be worth considering, but note this spell isn't concentration. So you can use it as an offense booster that won't interfere with your other concentration spells. A very decent alternative to fighting with two weapons.
Mirror Image: While fairly low level, Mirror Image is considered a top choice because it doesn't require concentration AND applies immediately. You'll never go wrong with using the spell like this, but it'll rarely be the best choice.
Otiluke's Resilient Sphere: This is THE perfect defense you want. And we do mean everything. It'll even block spells if you set the contingency correctly. In addition to working on everything, if the situation is particularly bad you can hold it up for several rounds while you do something like cast non-concentration buffs. The only problem is that it will break any previously held concentration, but that's a small price to pay for this kind of defense. Unless you're using Contingency for offense with Bigby's Hand, Melf's Minute Meteors, Haste, or Greater Invis this should be your default pick.
Polymorph: Surprisingly, this isn't a very good pick. Your concentration bonus will tank when using this spell, so unlike Otiluke's Resilient Sphere it may only be good for one attack despite the huge number of extra hit points. It's better than that spell for its duration and for getting you out of trouble that Resilient Sphere couldn't (such as being teleported into the middle of the ocean) so if you want defense as opposed to a mixture of defense/offense/all-around utility pick that one instead.
Stoneskin: For a concentration spell, it's not the best choice in the world. You're mainly using it for its ability to persist through several combats. It's better if you're fighting against a lot of high-accuracy/high-damage monsters like giants or dragons.
Telekinesis: A legal, but rogue choice. The idea is that you target yourself with telekinesis and then in subsequent rounds use it on someone else. This works out to an extra fifth level spell slot. If you're creative, you can use it defensively like Misty Step or Dimension Door to throw yourself out of the way of an attack (with a very good chance of it working, due to your low strength). Do note that targeting yourself on the first round will restrain yourself for at least one round if you win the check. Generally best to stick to Bigby's Hand, unless you really like this spell.
True Seeing: A decently useful, if very situational, pick for Contingency when used in certain circumstances. True Seeing is a lifesaver when you really need it, but the problem is its mere hour-long duration and 5th level spell slot. Throwing it out when you know you'll need it saves you on spell slots, spells prepared, and time.
). In these situations your biggest challenge is predicting the threat level and spending an appropriate number of spell slots. In less favorable circumstances, it's still often possible to justify a surprise round, or scream "Mirror Image!" so quickly cutting off the DM's description of the enemy that your character becomes known for his trigger finger. Otherwise, yeah, you probably only want to cast a single buff spell before entering combat (or none if you think your AC is enough). Depending on your party comp, and how well you communicated that you're playing a striker and not a controller, you might also be lucky enough to receive buff(s) from someone else first round.
