I can't decide between first person shooters or RPG's theyre hard to compare, but for the 360 I'd say either Oblivion or Halo 3, But I will say Oblivion, Halo 3 had the best campaign to finish the trilogy perfectly and its multiplayer along with the forge was awesome. But Oblivion because it was so expansive, beautiful graphics and world for a platform game in '06 and its DLC was superb especially considering what other franchises call DLC and what they charge for it.
So in conclusion, Oblivion.