Contains unpopular opinions for two particular films that many would probably put on opposite ends of the list as me. Please don't judge, me and my friend on the MCU Wiki moved here because the community seems nicer here in general and I hope my trust isn't misplaced here.
Guardians of the Galaxy
Avengers: Infinity War
Thor: The Dark World
Guardians of the Galaxy Vol. 2
Iron Man 3
Iron Man 2
Ant-Man and the Wasp
Captain America: The First Avenger
Avengers: Age of Ultron
Captain America: Civil War
Captain America: The Winter Soldier
The Incredible Hulk
Civil War is so low for being so dark, for technically ending in villain victory, etc. Which technically also applies to Infinity War, but at least IW was a fun ride, this really wasn't to me. The final battle here was not against the villain, but a heartbreaking battle between two heroes, which at LEAST may be fixed in Endgame. But I put above The Winter Soldier coz at LEAST there was something about Avengers fighting each other, and Spiderman provides comedy. There is almost no comedy in TWS at all (I think like, two moments), nor supernatural special effects, both of which are a huge part of why I love MCU.
The Dark World is so high for having a bit of comedy (mostly from Loki), and the awesome final battle between Thor and Malekith, and while I admit Malekith has very little personality other than being a generic apocalyptic villain, he felt compelling all the same. That movie always feels shorter than it actually is, and I think it's underrated.
"Above average" is probably from Ant-Man to Thor (MAYBE Iron Man 2), and 1-8 are my top favorites. Iron Man was promoted after watching it again, while Age of Ultron I think I demoted a bit. From Civil War to Hulk is "below average".
Endgame is only lower than Infinity War for the first hour being slower, but the rest was fun.