Here are the core Marvel films that I've seen ranked in order of which I enjoyed the most down to which I thought were worse than wet hot turds in the sun.

I maintain that Iron Man 3 is the best one probably because it's the least MCU of all the films: it's got a weird Shane Black screenplay; there's less superhero punchfests than usual; the kid is hilarious; the henchmen are funny; Guy Pearce is sexy; the villain is moderately more interesting than usual.