Garry's Mod has about 2500 hours, so I guess that's my favourite.
Red Orchestra 2 is 100% the best FPS.
MnB Warband is a helluva timesink.
Outlast is the best horror game.
Prison Architect best management/building game.
Huniepop is the best game to have a wank over, with Tomb Raider being a close second.