Call of Duty: United Offensive (estimate)
Gears of War 2 (also estimate)
Pokemon Platinum (170 hours)
Counter-Strike: Source (As of right now, 194 hours)
Left 4 Dead 2 (203 hours)
Team Fortress 2 (My Steam page says 81 but I know there's more... reset my stats, too, so I can't go in and count)
I'm sure there are more, just none that I can be completely sure of. I never spend too much time on just one game. I try to spread it out.