According to Steam, over 600 hours in Team Fortress 2. Been years since I played, but still no other of my Steam games has come close to beating this score. The closest is Mount&Blade Warband with around 360 hours or something like that, and maybe Mass Effect trilogy if I took all 3 games together.

However, there were some childhood games that I played a lot (Gothic 1 and 2, Lego Star Wars Complete Saga, Oblivion). No Steam back then, just classical CD, so I can't tell how many hours I put into them in total, but I know they would be very high on my list, especially Gothic games.