Oblivion is terrible. [...]
Skyrim is good when you finally calm down and accept it for what it is.
I'll never understand this. Not one thing in Skyrim is even as remotely entertaining as Oblivion's DB questline or the shivering isles expansion. Magic was even more neutered and useless, setting took an even blander turn with the snownigger shit and how out of place certain things felt like the magic guild.
And you mentioned dungeons.. I'll leave this here
Oblivion is technically more complex than Skyrim in a lot of ways, but (for me at least) absolutely none of it comes off properly. To put it bluntly, Oblivion is still trying to be an RPG, and it doesn't work because it's an absolutely terrible RPG. Skyrim knows it wants to be an action game with stats, and it just about succeeds IMO.
I can't agree with you on the setting. Skyrim's setting provides a fine backdrop for the player's adventures, is visually appealing, and individual dungeons provide memorable snippets of lore and worldbuilding, even if it never feels like it's tying together into a cohesive whole. Skyrim shows much more respect for the TES setting than Oblivion, I'd argue, and it does genuinely feel like it's connected to the world established by Daggerfall, Redguard and Morrowind. Oblivion is just an ass-looking version of Middle Earth with nightmarishly garish colours, with no relation to the Cyrodiil we'd read about in the other games, and no interesting factions (I didn't like the Dark Brotherhood questline personally, so that leaves me with the endless parade of Counts and Countesses who do nothing at all).
That dungeon comparison chart doesn't translate into the dungeons being fun to play. To illustrate the point, Daggerfall's dungeons dwarf anything in Morrowind, Oblivion or Skyrim in terms of both scale and complexity, but they're often no fun to play through. Skyrim's dungeons are more like rollercoaster rides (terrible analogy but it's the best I can think of) through a pre-planned series of traps, enemy ambushes, visual setpieces, usually some kind of backstory to uncover, and almost always some kind of boss fight at the end with unique spells or weapons or dragon shouts. Oblivion's dungeons are more expansive with more to explore, but exploration is no good if it's the kind Oblivion offers - trudging through samey copy/pasted corridors full of the same three types of HP-bloat level scaled enemies for some uniformly shitty loot.
Oblivion was decline but every step Beth took was made for the worse, every single time. They haven't made a better game than oblivion since oblivion, just as they haven't made a game better than morrowind since morrowind. Oblivion was a mediocre game and it caused a lot of butthurt on the codex, mostly the morrowind rapefugees who invaded, but as far as AAA garbage goes it's just that, average. It's nowhere near as bad as modern Beth. Writing is bad, but not -I want to rip my eardrums from hearing such insane shit- bad like Fallout 3 which had things like the kid city, the grown kid city, the fake vampire undergrounds, that infamous three dogs "intelligence" check etc.
Don't worry, I'll make no attempt to defend any part of Fallout 3's writing whatsoever. I think it's better than Oblivion because the dungeons feel closer to Skyrim's sort of dungeons I described above, the world map can be interesting to explore (especially the downtown DC area, which is very visually striking) and the combat was wisely made more FPS-like than RPG-like (though it's still fucking shit). VATS was a bad design decision and exists purely to cover how unplayable the regular combat is, but it's a still up from Oblivion's HP-bloat nightmares.