My top 5!
1: Team Fortress 2 (319 hours)
2: Garry’s Mod (267 hours)
3: Planetside 2 (124 hours)
4: Counter-Strike: Source (106 hours)
5: TrackMania 2: Canyon (105 hours)
Let me just point out, though, that if I took my playtime in all TM2 games plus TMU and put it together (excluding my time in TMN and when I just used ManiaPlanetLauncher.exe), that totals to 295 hours of TrackMania. Yes, I am addicted to those games.