No. 3: Iron Man

Without the success of this movie, none of the other eleven movies in the MCU would ever have happened. I honestly can't fine anything wrong with this movie. Not even the one-time role of Rhodey by Terrance Howard was a negative. Obviously, Don Cheadle has improved on the role.
Seeing Robert Downey Jr. become this famous character created by the great Stan Lee, a character that has been loved and hated, and make him the most beloved action character in a long while was utterly mesmerizing. Downey Jr. is Tony Stark. There is no getting around that.
This film was given a major boost by keeping things pretty darn close to the comic books. The origins of how Tony Stark becomes Iron Man, a man who realizes that he is doing wrong by selling weapons, was as compelling on the big screen as it was on paper.
Iron Man is one of the two centre-pieces of the MCU and had this film not captured the hearts of so many fans, the universe that grew up around him might never happened.