CoD4, trust me, multiplayer will keep you going for ages.
Bioshock is good, but not the followup to system Shock 2 it should of been. and Crysis... well, the engine is great... the game... i dont rate it in the slightest....
Please refrain from using text speak in these forums (i.e. u instead of you). It's annoying and pointless. You'd know it's not allowed in this forum if you'd have read the rules that you're forced to read when you first come here . =]
COD4 is probably the best PC game out right now.. It has intense single AND Miltiplayer.. One of the best Shooter story's I've ever seen. its an amazing game.
Not really any of those, but if I had to choose I'd say BioShock - you could make a movie out of a game as deep as that one (no pun intended, but appreciated nonetheless).