The best game played in my life was Mafia (on PC). I played numerous nice games since then but Mafia will have a special place in my gamer's heart I finished it 4 times (don't worry i'm not a game addict , i just love that game)
The original 3D version of Castle Wolfenstein on the PC changed my life and the world of gaming as it was really the first (or the first good) 3D 1rst person shooter!
For me it would be Team Fortress Classic. I must of played that game for 4+ years participating in clans, leagues, etc.