Iron Man 1-3
2. Seen 13/15. Unseen: Hulk, Guardians 2
3. Well, I didn't dislike them, so - no?
4. Unlikely. The two Thors were pretty much the bottom of the barrel.
Iron Man probably benefited from coming out before that whole MCU/DCU/whatever thing became just an annoying, noisy mess. With the exception of Guardians, I've seen all of these films only once, and I think it's safe to say that a rewatch would do none of them any favors.