By game universes, I mean look at the game WITHOUT the gameplay mechanics or storyline. Try to look at it purely from a Virtual Reality perspective, and which worlds feel the most realized, complete, and aesthetically pleasing.
- Mass Effect
- Red Dead
- Gears of War
- Dead Space
Those are my top 10 (not ordered)
EDIT: Also if a game world has had such an impact on you where it made you want to do something you otherwise normally wouldn't have done (i.e. Skyrim got me into mead making) like Bioshock making you want to visit an aquarium, Borderlands getting you into comic books, or Condemed getting you into beating hobos with a steel pipe, I want to hear about that!