If you want a good through-line for the movies that are part of the canon marvel Comics Universe for film, and if you just want to see good movies, i would watch:
*The first Iron Man movie (I didn't like 2 or 3, and they're not that necessary to see to know what's going on in the Avengers movies)
*All three Captain America films (so far the highlight of the Universe--all three films are phenomenal, and the third one is really an Avengers movie)
*The four Avengers movies (the core of the universe--although they are all too long and have too many endless battle sequences)
*The two Ant-Man movies (the first in particular is the most underrated gem of the MCU films)
*The 2 spider-Man movies with Tom Holland (both very well done, and both the only Spider-Man movies connected to the rest of the MCU series)
I would skip all the Hulk films, and also Iron Man 2 and 3, unless you just love RDJ to pieces (I do not). Dr. Strange and the two Guardians of the Galaxy movies tie-in very closely to the Avengers movies, but I was not a fan of any of them.
Of the films that are not considered fully part of the MCU continuity but still feature Marvel superheroes, i would say to watch:
*Spider-Man and Spider-Man 2, both with Tobey Maguire (the second of these is particulatry excellent)
*X-Men and X2, its sequel--and X-Men: Days of Future Past is okay
*The Wolverine (set in Japan) and Logan (very sad and melancholy, but in a good way)
In this non-MCU continuity, I would then definitely skip:
*All three Fantastic Four movies (just awful)
*Spider-Man 3 and the two Amazing Spider-Man films with Andrew Garfield (none of these are part of the MCU continuity)
*Daredevil with Ben Affleck (just awful--maybe the woirst of all the Marvel movies with the three fantastic Four movies in a possible 4-way tie with it)
*All X-men movies except the first and second (the last few have been terrible)
*Deadpool and Deadpool 2 (only funny if you're a teenage boy)
*Wolverine (although Liev Schreiber is incredibly sexy in it)
*Venom (disgusting, and only so-disgusting-its-fun if you're a teenage boy)