Counterstrike and COD4 tie for the first spot. Counterstrike was great in its simplicity and accessibility. COD4 is simply, in my opinion, the most polished and rewarding game ever made. Yeah, I'm a fanboy. Suck it.
Forza 2 and UT2003/4 tie for the next spots, with honorable mentions going to Company of Heroes, Soldier of Fortune II, and, of course, Duke Nukem 3D. Stripper tits rock.