Though opinion may change over time in future viewings this is how I would rank them for now:

1) The Avengers

2) Captain America: The Winter Soldier

3) Guardians of the Galaxy

4) Iron Man

5) Captain America: The First Avenger

6) Thor

7) The Avengers: Age of Ultron

8) Iron Man 3

9) Iron Man 2

10) The Incredible Hulk

11) Thor: The Dark World

Guardians of the Galaxy.

All the first movies for characters are the best imo. While some aspects of the sequels are okay, they devolve into long battles I care nothing about. It's at its worst in Thor 2 (yes we get it they are brawling through portals), Cap 2 (are there bullets left on Earth?), Iron Man 2 (Let's make viewers watch Tony fly around and make drones crash into things for ten minutes), and I know everyone will hate this but the Avengers (Let's develop a nonsensical plot and then have a nonsensical battle that defies all tactical sensibility).

Iron Man 3 tried to break out of this, but the last scene was still a festival of explosions that got dull. In GotG they learned or at least accidentally stumbled on the RotJ formula by appropriately clipping between theaters of action that varied from the larger view of the conflict to the very personal, and having some sense of a plan/goals that made the action seem necessary.