Iron Man, just because he has the snappiest comebacks, best general banter, and a wonderful sense of style. Thinking back to his and Banners epic bromance, it was Stark who really brought banner out of his shell. And the fights between him and Cap? Tony Stark didn't EXACTLY start the fights, be he definitely set the tone for there grievances. Also, he fights with science.
Honorable Mention to both Hawkeye and Black Widow though. For 2 characters without movies, and without crazy abilities of the others, they still managed to be constantly useful, interesting, and badass. I genuinely wasn't sure they could stay relevant when gods and monsters started breaking shit, but they did. Now, Id watch a movie staring either of them.