Only going to list games with 200+ hours:
Code:
Counter-Strike: Source
2,178 hrs on record
War Thunder
1,005 hrs on record
Grand Theft Auto V
723 hrs on record
Arma 3
718 hrs on record
GRID 2
469 hrs on record
ARK: Survival Evolved
418 hrs on record
GRID Autosport
315 hrs on record
Worms Clan Wars
247 hrs on record
League of Legends
236 hrs on record (but far far more on the non steam version)
Warhammer® 40,000™: Dawn of War® II
231 hrs on record
PAYDAY 2
209 hrs on record
EVE Online
206 hrs on record