#5 Oblivion
The size of Oblivion makes Morrowind seem like a playground—which isn't to say that Morrowind is small, only that Oblivion feels that much bigger.
Despite the fact that Oblivion lacks the bizarre architecture of its predecessor, it makes up for it with the addition of Oblivion Gates, which are pathways to another dimension that looks a whole lot like hell.
If you didn't think Oblivion was a big enough place to explore or missed the weirdness of Morrowind, the game also saw the Shivering Isles expansion which added… well, the Shivering Isles.