To me it seems the MCU did more building off itself with later movies than with its earlier ones (there’s still a ton of building, but just… not as much?) Or maybe I don’t remember it as vividly since I only saw each movie once, so the older ones would be older memories?
I saw Black Panther only after Infinity War, it wasn’t necessary for my understanding of that movie. Captain Marvel can be skipped in that you don’t need it to understand other things (but it was fun on its own).
I never liked the Captain America stand-alone movies, so only ever finished Civil War. I don’t think I missed too much that I couldn’t piece together, but I am less attached to Captain America than others seem to be.
The question of which are important to see is a bit inaccurate – they’re all important, but which ones are most important based on the constraints you have? If you can only see one movie, watch infinity war. If you can watch 2, then infinity war and age of Ultron, etc., and it builds from there.
My opinion:
These are ranked in order of importance. Determine how many movies you have time to see, but then watch them in order of their release, reading the synopsis of any movies that came out in between that you don’t have time to see:
All 3 prior Avenger’s movies
Civil War
Guardians 2
Thor 3
Ant-Man 2
Dr. Strange
Iron Man (probably #1, but I think any of them would suffice in a pinch)
If you have time for more (again, as them in based on release date):
Guardians 1
Thor 1
Ant-Man 1
The other Iron Mans
Still more time?
Might as well watch the rest of them. I would prioritize Spider Man, Black Panther, and Captain Marvel last (I haven’t seen spider man, the others don’t contain anything that you can’t figure out from context in the other movies)