What should I buy?

I heard Trasistor is pretty good.

You can always spend all in keys, then save it for a better deal.

Antichamber (1k) and Star Wars Battlefront II (2k). This is what I'd get. Just a suggestion :)

I think you can get Torchlight 2 for 3 keys, it's a very good game :D

Thanks for the suggestions. After exploring the Steam Store some more, I got an interest in Batman: Arkham City, Metro: Last Light, Skullgirls, and Tomb Raider. What about narrowing my options down from there? Rank them 1-4, 4 being worst and 1 being best.

Natural Selection II 2 Keys /
King Artur's Gold 2-3 Keys /
Civ V 2Key /
Killing Floor 1,5-2 Keys /
The Last Remnant ~2 Keys

There are people who sell Sonic Generations for 2 keys

Beasts of Prey looks pretty fun, and it's a 19 dollar game for 3 keys. :)

split some 4 packs. payday 2, killing floor, etc.

Far Cry 3, Batman Arkham Origins and so on.

Just start a Steamtrades thread with the title "[H] 3 TF2 keys [W] Offers", you will be surprised at how many offers you get.

I noticed Dishonored on your wishlist and I can confirm that it's an amazing game. You can probably get it for 3 keys now, but then again, you could always grab GOTY during a sale for 4-5 keys (which is what I'd advise you to do). Thief 2014 is also going around for 3-5 keys so you could probably grab one from someone who has the AMD Promo card. Bioshock Infinite is also 3 keys I believe.

Tomb Raider and Last Light are about 2 keys each and you should be able to grab Mark of the Ninja, Guacamelee and Limbo for a key each. So yeah, take your pick. :P If you'd like I could use my inside knowledge on trading to help you get the most bang out of your buck, if you do, just send me a friend request. :) Hope my post was of some help.