Scoobes on 14 November 2013
Probably one of the Elder Scrolls games as they never end.
I clocked over 400 hours in Oblivion, 250 on Skyrim. Other than that it's probably Dragon Age: Origins at around 350 hours.
If you count genre mash-ups then Deus Ex is probably pretty far up there although actual numbers are difficult since it released before Steam stats. I played through it about 5 times when I got it and have played through at around once a year since I got it (depending on if a new mod was released). That's not including the time spent playing the multiplayer portion.
