Here's my brief take on most of the "main" villains, by film:
-Iron Man
-Obadiah: Not great. Kind of standard "comic book" villain type. But it was the very first MCU film, and he set up the Iron Man origin just fine.
-10 Rings cell leader (don't know his name): More of a generic "type" of villain, but still worked well.
-Hulk: Blonsky was meh. One of the lower tier villains for sure.
-Iron Man 2: Whiplash was fine. He had an understandable motivation and played the role well. The problems with this film are not the villain.
-Thor: Loki was good in this, but not as good as he would become.
-Captain America: the First Avenger: Red Skull was pretty good. The source material had problems in the modern age, but I thought they executed him well enough.
-Avengers: Loki was really good. Not going to mention Thanos since he wasn't really in it.
-Iron Man 3: Trevor's Mandarin was FANTASTIC. The "real" Mandarin was decent. His motivation and who he became were actually pretty good. It's just that he became kinda cartoonish by the end.
-The Dark World: As I understand it, Malekith's motivation was the survival of his people, combined with an eon's old grudge against Asgard. I get it. The bones are there for a good villain. Just not executed particularly well.
-Civil War: Bucky and Hydra as a whole were great.
-Guardians: Ronan was great. Kind of one-dimensional as a zealot, but it worked. Nebula was good as the henchwoman.
-Age of Ultron: LOVED Ultron.
-Ant Man: Put this in the small pile that didn't quite measure up. But then again, Cross did fit the tone and scope of the film, so there's that.
-Civil War: Zeemo was pretty solid.
-Doctor Strange: Dormammu was pretty generic, but I thought was fine in context of this film. Kaecilius wasn't bad either as the zealot deceived. He was underdeveloped for sure. But given that the focus was on Strange, and there was a lot of ground to cover there, I get it.
-Guardians 2: Yeah, Ego didn't work for me.
-Spiderman: Homecoming: Vulture is one of the best, which surprised me.
-Ragnarok: Hela was surprisingly disappointing for me. I get that conquering is just part of her nature, but...I dunno. She just fell flat. And how ridiculously overpowered she was made her seem too cartoony. This was a great film despite Hela, not because of her.
-Black Panther: Killmonger was okay. I thought Klaw as the "b" villain was great though.
-Infinity War: Do I even need to say?
-Ant Man and Wasp: Probably better villains than the first Ant Man, and they worked in context, but ultimately aren't that great. They just kinda moved the story along.
-Captain Marvel: Eh...
-Endgame: Both versions of Thanos were great.
-Far from Home: Mysterio was pretty good, overall.
-Black Widow: Drakov was pretty good. I agree with most that Taskmaster was a disappointment and huge missed opportunity. (As a side note, it's interesting that Marvel seems so in love with Taskmaster as a property and then made such a mess of her in the film. The Marvel edition of Villainous, for example, has Taskmaster as one of its 5 villains in the game. And the Disney parks feature Taskmaster in their Avengers showdown show in Avengers Campus.)
-Shang Chi: The Mandarin was a hot mess. This entire film was a big missed opportunity that set up a huge payoff in terms of the dynamics of family, and instead focused on a big CGI battle with dragons.
-Eternals: Who was the villain again? Meh. Don't care.
-No Way Home: No comment. They were good. But this film is somewhat unique in the MCU in many respects, and the villain lore and development is another such area where it stands alone.
-Multiverse of Madness: Another where it is hard to comment. Wanda as a villain was good, both because of the tone of the film, and because of the huge arc she has developed in other MCU properties. But the film was such a stupid mess that I feel the best parts of her get lost. Parts of her were written well, while other key aspects were just put out there and taken for granted that the audience would overlook/accept without giving any thought.
EDIT: Love and Thunder: Gorr was done pretty well overall, although I feel there was still missed potential that was likely left on the cutting room floor. Perhaps best villain of the phase 4 films.