Hmm...
1. Minecraft - Probably around 200-300 hours in 2012 alone.
2. Team Fortress 2 - 150 hours.
3. Skyrim - 80 Hours, perhaps more now that I have the DLC and some refreshing mods.
4. Payday: The Heist - 40 hours, give or take.
5. Killing Floor - 30 hours, mostly spent unlocking stuff during the holiday events and getting my last few perks up to level six.
6. Saints Row: The Third - 20 hours, though more to come now that I have all of the DLC to toy around with in co-op.
A lot of these are estimates. I have no real way of tallying Minecraft's playtime outside of averaging my daily/weekly excursions. Likewise, the other games have some playtime logged through Steam, but I often play in Offline Mode so the stats aren't quite right.