Minecraft, obviously :)
Beyond that...Civilisation (4, now, though for a long while I played 3), but that's more in the nature of the game than anything else.
Oblivion technically, but only because I keep coming back for a day or two at a time to see if it really is as bad as I remember (and also to try and get through the story so that I can actually know what happens. Otherwise I can't roleplay my character's descendent in Skyrim)!
Now, Morrowind would make this list, right at the top, but I actually only got it quite recently, and so have only played it through once. Now I'm letting it fade a little in my mind before I come back to it, so so far it doesn't make the list. I think it will in a few years, though...
Is Morrowind that much different that Oblivion? I was in another thread talking about how I liked Oblivion to a degree but there was a lot they could have done better, so someone immediatly recommended Morrowind. It's currently in a mail truck on its way to me.