Mine would have to be both Fallout 3 and Fallout: New Vegas. Both are superb games (in my opinion at least) that you can spend countless hours in, and not mind doing so. Right now, I'm working on The Elder Scrolls V: Skyrim, and I'm thoroughly enjoying it. If I had to say, when I get the 100% in Skyrim, Skyrim would be MY #1 game that I enjoyed getting 100% on