Vanilla World of Warcraft it may be gone but the legend will live on forever
BG2, followed closely by Starcraft, followed closely by Tie Fighter
Counter Strike. I wasted my childhood away on that game and De_Dust.
Probably Portal 1 + 2, although I really enjoyed Deus Ex Human Revolution, Far Cry 3 and Bioshock Infinite. I'm probably going to replay Deus Ex if something interesting doesn't come out soon.
Hands down, Skyrim. About the biggest single player experience there is. Have been playing it for almost a year now. Can't get sick of it.
Men of War: Assault Squad 2
Single player is horrible, but multiplayer blows CoH 2 out of the water. Not even close.
Mount & Blade: Warband. My most played game ever. Every couple of months I start playing it again and suddenly I have played 50 hours in a week haha.
technically skyrim has more hours on steam but warband was the highest for a long time and i like the game much more than skyrim or any other bethesda game since morrowind