I voted Morrowind.
First off, let me say I played Oblivion first, then Morrowind, then Daggerfall, then Skyrim.
Oblivion is a lot of fun as a game, but it has some very bad mechanics (level scaling for example) at the same time. Also copy+paste dungeons. It being my first foray into the series, I still love it despite it's shortcomings. One thing I don't like so much looking back is how "mainstream fantasy" Oblivion went- you've got Minotaurs, imps, mithril armor, etc.
Morrowind is the best one, in my opinion. The locations and enemies were unique, but there were still things you could relate with history (the Empire's obvious Roman influences for example). I liked the diversity of skills and how the weapons skills were broken up into several different things (rather than just Blade and Blunt for example). The weapon variety was also good, with spears, throwing knives and stars, darts, crossbows, katanas, tantos, sabres, etc. I also like the non-leveled loot, and the rarity of high-level loot like Daedric (I recall there is only one full set you can get in the game). The unique items were also fun to collect and almost always had unique models. In Oblivion you had forts, caves, and Akaviri ruins; in Morrowind you had more mysterious and exciting locations like Dwemer ruins and Daedric temples.
Daggerfall is okay, I think. I honestly don't understand why people love it so much. I guess the thing that gets me is that everything is just same-y as a result of most of the content being randomly generated. All the cities look the same, all the people look the same, all the dungeons look the same, etc. A big part of The Elder Scrolls was exploration for me, and interesting exploration is impossible with the boring and drab landscape in Daggerfall. It's almost entirely flat and devoid of content. The only variety came from reskins. All the quests I did involved "go kill this monster in a house" or "go kill this monster in a dungeon". And then the game ultimately just turned into a dungeon crawl with me just going to towns in between to sell loot. For me, it was repetitive and boring after maybe 20 hours of gameplay. I did like the variety of skills, however.
I think Skyrim is good, but I'm disappointed with where they've taken some of the mechanics. I was excited for Fallout-style perks, but got what I think is a pretty crappy system instead. There are barely any skills now as well. I like the carriage fast travel system, but don't like how you can't travel via boat like in Morrowind. I would have preferred mostly non-scaling stuff like Morrowind, but the scaling now is at least better than Oblivion. Magic is generally a disappointment as far as variety goes, but the spells that are in the game do look nice. There are some interesting enemies, but mostly I find I'm just fighting bandits, undead, or Falmer. Dragons were definitely a fun addition, and I think they were done well, but at the same time it can get repetitive. I played this much less than Oblivion or Morrowind, and I think it's because there aren't as many roleplaying opportunities as in previous games. I guess overall I would say it is a good game by itself, but doesn't compare well to Morrowind. I do like how the Empire is more Roman-like again, like it was in Morrowind.