Different ideas of direction. I think Cap had the best cinematic director, as in it looked fantastic, great cinematography, Thor had Brannagh who is a fantastic Actor's director and brought out the best of all the actors, Favreau and RDJ brought that great off the cuff impro style that made Iron Man feel really off beat and different, so it probably had the most charm and personality, and I think Hulk probably had the tightest, well paced action scenes.
I think comparing which one is 'best' is kinda silly really, because it's horses for courses, same as the comics. In all these films, the characters are totally brought to life and are put first, before all else, and certain characters resonate better with certain people. That's what's so great about the MCU, it brings many different types of people into the fold. Apart from all the fanboys watching these movies, the fact that it has brought a comic book sensibility, regarding character and continuous narrative, to the film, it's essentially turned members of the GA, normal film goers, into comic readers. That is the merit on which I believe the MCU should be praised.