Out of curiosity, do you literally mean the Avengers in particular, or the Marvel movies in general?
If it's the former, then my answer is Daredevil. If the latter, though... it's kind of a dead tie, really.
Basically, I think that part of what makes Daredevil so enjoyable is the same things that (to me) made the single-character Marvel films like Captain America or Thor more enjoyable and impactful than the Avengers movies; focusing on a consistent tone and creating a very distinct 'flavor.' Iron Man was basically a straight up sci-fi tech wet dream, complete with snazzy armor effects and lots of flying missiles and energy beams. Thor was more science fantasy, with more dramatic dialogue, grand scenery (Well, in Asgard, anyway,) and hand-waved 'Science And Magic Are Sorta Same-y Here!' things to explain all the WTF things the Asgardians in general, and Thor in particular, were able to do. Captain America always felt a bit like a spy thriller (albeit a louder, more explode-y one,) with the one man fighting forces of evil through awesomely skilled applications of violence; Winter Soldier in particular sometimes felt like Bourne on super steroids. Guardians of the Galaxy was almost kind of a blend between sci-fi and science fantasy, IMO at least, but it stuck to that tone consistently. By the same token, Daredevil was a darker, more grounded series about a street vigilante, and the action, enemies and plot points largely center around that, with only brief easter eggs alluding to the wider universe.
Avengers, though, had the perhaps unenviable position of taking these characters from all these different genres of setting and style, and sort of mashing them all together in something that adequately fit everyone. Unlike the comics, however, or even the various animated shows, they don't have the luxury of exploring multiple venues and plots over a longer period of time- they can't have a Thor-centric episode of Avengers, a Captain America centric one, etc- and are thus left trying to take smaller elements from different heroes, and cobbling them all together. Iron Man brought in the use of Nick Fury, Captain America brought in the Tessaract, Thor brought in Loki, etc.
The result, I feel, was a movie that was entertaining in a 'Dear God Eyegasm Explosions' popcorn sort of way- I do enjoy the Avengers movies- but still ultimately felt more generic and homogenized than the films that had led up to it.
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