Please presume ALL films based on Marvel characters could be included in this question, not just those in the "Marvel Cinematic Universe".
One other caveat for me; I haven't seen every Marvel film. I've yet to see IM 3, Thor 2, Guardians and Ant Man. So my answers can't factor in those films. Never saw the Garfield Spiderman flicks either.
Anyway, from what I've seen:
MOST UNDERRATED: The Incredible Hulk. Still say Norton was the definitive big screen Banner. Lots of great homages to the TV show, a strong villain in the Abomination, and I really bought the relationship between Banner and Betty. Was it perfect? No way; but it really deserves a better place in history than it's getting, I think.
MOST OVERRATED: Probably the original Avengers. Not that it's a bad film by any means; I liked it and left the theater happy. It's certainly better than the sequel. But when I really think about it, the villains (besides Loki) were remote and hollow, the dialogue was way too "snarky" a la Buffy, and overall it just doesn't stick with me like I thought it would.