Why not make this a poll.
Definitely Morrowind to Oblivion. With mods both can be made much better. But without mods definitely Morrowind to Oblivion.
I would say Morrowind to Skyrim since both of them has in-game depth, which Oblivion doesn't have one bit of imo.
This seems to have nothing to do with the question. But I 100% disagree with the quote above, and if someone played Oblivion with an open mind I think that would be clear, imo. Since it may be irrelevant to this thread I won't say any more on that.
In addition to these another big leap forward in my opinion was Skyrim's water....
Lot's of wonderful graphical improvements with Skyrim, and water is one of them. And not just the graphics, I love the implementation of water currents.