Toss up between Fallout 3 and Skyrim. I reserve the right to change that answer in the future, as I haven't played Dark Souls yet. --- I only go to Solstheim to get things done. Manly things. Like ripping Netches apart with my bare hands, or pummeling snide Dark Elves into Flin. Hail HYDRA!
Realistically, I'd go insane playing any game over and over without any other to play.
That being said, if I was to go insane playing only one game and that's it, you can be damn well be sure my sanity would snap over Resident Evil 4 --- http://i840.photobucket.com/albums/zz329/TehAwsumDude/dealwithitspider.jpg http://i840.photobucket.com/albums/zz329/TehAwsumDude/sonicderpp.jpg