Meh, I'll throw in some slightly dissenting opinions, just for flavor's sake and because I can. I personally did not like Bioshock at all. I found it shallow and not remotely close to the game it was pitched as the spiritual successor to (System Shock 2), and the PC port seemed to be a typical afterthought for the console version. COD4 wasn't bad, but it was also an extremely oversimplified shooter on rails. I did like the storyline though, and there is a non-first-person mission that I absolutely loved and was my favorite part of the game, but I wont spoil it.

Personally, I'd recommend Crysis. It is what it is, it's a beautiful, straightforward, fun shooter on rails. Storyline is dumb as can be, the gameplay is terrific, controls are crisp and it's clearly built FOR a PC and WASD/mouse, the atmospheric audio is immersive, the spoken dialogue is funny (horrible accents), and the visuals are stunning.