Well, besides oblivion I would say Morrowind

. But I suppose that's not what you want to hear.
So next after oblivion is tough to decide really, I think Gothic 2 is a close one but that hasn't got the re-playability I look for in good games. Though the first and second time I played it it was like a god

. Fallout 1 is also a close competitor at the top, simply an excellent game.