I was looking on Steam, and if you look at your gameslist in the community tab, they sort the games by how long you've played them.
Here's my top ten:
Fallout New Vegas at 268 hours over three playthroughs.
King's Bounty the Legend at 106
King's Bounty AP at 103
Just Cause 2 at 76
Dota 2 at 61
Space Pirates and Zombies at 57
Deus Ex HR at 54
Dredmore at 53.5
Blocks that Matter at 49
Orcs Must Die 43
My off steam games that probably kick the rest down a notch are:
Fallout 1/2/3, BGII (4 runthroughs), Torment (2 runthroughs), NWN 2 (a lot of runthroughs, maybe 7), Dota, and WoW
It's notable that I didn't pay over 10 for any of my most played games on Steam (which makes it hard to justify buying wii games).