Which Marvel movie do you dislike the most?...
Spider-Man 3, X-Men: The Last Stand, X-Men Origins: Wolverine, Iron Man 2, Iron Man 3, The Amazing Spider-Man, The Amazing Spider-Man 2, or Thor: The Dark World?
Of course, if there's any Marvel movie (that you dislike) that I didn't mention feel free to add it as a response to this thread.
Anyway, for me it's a tie between Iron Man 2 and Spider-Man 3 because both of those movies sucked hard. Say what you will about Shane Black's Iron Man 3 and Marc Webb's Amazing Spider-Man Duology, at least those movies never turned into a musical with singing and dancing (a'la Spider-Man 3) and they were self-contained stories that were not a commercial for The Avengers like Iron Man 2 was. Of course, Amazing Spider-Man 2 showed you that Sinister Six technology at the end of the film (after Gwen Stacy's funeral) but that was after the story was done.
TASM 2 didn't put the story on the back-burner to have a commercial for the Sinister Six movie.