Thor vs. Iron Man is my flat-out favorite CBM fight of all time. It was so well crafted that you got a full sense of the power both of them possess, yet it managed to show just who was the boss. Thor putting the hammer down on Cap was the icing on the cake. That fight was one of the longest one-on-one battles in any MCU film, if not the
longest, which really I appreciated. Marvel and Whedon definitely pulled out all the stops on that one.
I was going to suggest adding the Hulk/Loki smackdown, but that wasn't a fight, it was a thrashing.