Skyrim, Skyrim, Skyrim. I've had that game for ages and I still find new stuff to do in it. I have more characters than you can shake a stick at. It's literally the only PC game I play anymore because I'm so busy.
In terms of handheld games, I'd have to say that the 3D remake of Ocarina of Time is the one I play most often. Blazed through it a zillion times, and I'm now halfway through a 3-heart-challenge Master Quest run.