I can't quite pick out a definitive favourite but here are a few I really enjoyed.
1.Fallout 3-bloody fantastic game, really immerse.
2.Forza Horizon-The Best licensed soundtrack in a game, awesome feels.
3.GTA San-Andreas. It's really impressive considering when it came out.
I'm reallllllly wanting to try Far Cry 3 though, and I have a feeling that Just Cause 2 would make the list if I gave it a try. Alas without and Xbox 360 or Ps3 I'm going to have to hope upon hope they make a Wii U release. (still don't have that either, but I'm strongly considering it.)