I've started a new playthrough of Oblivion after years since my last one and I have to say that, even with all the problems with the game, it is my favorite TES in the whole series.
Daggerfall doesn't do anything impressive in my view, aside from the simulation of a fantasy world and perhaps the player leveling system. The dungeons are too big and monotonous and the quests are likewise monotonous, yet there's no incentive, besides leveling up, that drives me to keep going. The setting is as generic as it gets, most of the good stuff would be introduced in Morrowind. It has the highest "RPG to Action game" rate in the series, let's say 90% RP to 10% action. Which shows especially in the main quest.
Morrowind is a walking simulator that also gets somewhat monotonous fast especially with the annoying combat system, but at least the setting is interesting and the player has the most freedom of any TES game (spells, enchanting, etc). The factions are interesting but their quests, not so much. The rest of the side quests make the setting stand out, but aside from that they're not particularly interesting. The story behind the main quest is probably the best of any TES and it has the best villain, but the quests themselves are nothing to write home about. The "RPGness" starts to decrease in favor of the action, let's say 70% RP to 30% action.
Oblivion has its issues like the enemies and items' leveling system and the uninspired setting. The radiant AI has its ups and downs, but at worst it can be pretty hilarious and at best it provides liveliness to the world. The player leveling system is better than Morrowind, this is where I find the streamlining to be good. The setting of the main game is rather generic on the whole, but certainly not as generic as Daggerfall, especially when you take the main quest into consideration (more on that later). The factions have the most interesting quests of all the TES games, at least I found them to be the most fun. Side quests likewise were also pretty creative and fun for a TES game. The main quest is interesting and later on starts to show the uniqueness of the TES world, to the point that it becomes Morrowind tier in terms of goodness, and I also found the main quests more interesting than Morrowind's. The trend of turning the series into action games continues, let's say 30% RP to 70% action.
Skyrim is mid. It's the most mediocre game that doesn't do anything better than the previous games. The enemies and items' levelling system is better than Oblivion but not as good as the other games. The setting on the whole is better than the other games but nowhere near the level of Morrowind. The faction and side quests are worse than Oblivion but not as bad as the other games. The main quest is not as good as the other games, except at the end when you enter Sovngarde. The worst part of the game is that, If you let yourself, you'll be the leader of every faction without the skills to back your position, like being the Archmage with only knowing how to cast novice spells. Skyrim is barely an RPG, let's say 10% RP to 90% action.
Overall, I think I enjoyed Oblivion the most to be honest. It's not as dumbed down as Skyrim, but it also doesn't have the monotony that I found in previous TES games. The quests are fun and I enjoy the whole experience more than the other games.