Fable or Oblivion?
Did I forget something? I know people are going to ask about Baulder's gate. Not in the same class, IMO.
Playlist: Mlb 2K8, Wii Sports, Wii Fit (!!!), Viva Pinata
(You'll notice that ever since I beat Oblivion, I've been more of a "casual gamer". That's Oblivion for you. That's about as hardcore as it gets in the world of one player RPGs.)