Probably Killzone 2

and im not saying that because i have a ps3, i play most shooters on the PC but Killzone 2 has this bleak, grim look to it that faintly reminds me of the legendary Quake 2: Industrial military wasteland with non stop action and anthropomorphic enemies that allows you to really hate them. it truly makes you feel like you're in the middle of a real warzone even though it's technically a sci phy game.
the battle outside visary's palace is absolutely legendary, multiplayer was extremely solid as well and the graphics some of the best this gen
close seconds: Bioshock, Deus Ex HR, Killzone 3 maybe Vanquish







