Probably not popular, but my favorito one por far is Steve (Captain America)
I adore his back-story and personality, and believe it or not, I really relate to the guy.
Steve is that person who tries to be the best he can be, even though he had so much getting in his way. He kept on believing that he could do good in this world, and even though he got his wish, and great power, he stayed that 'awkward kid from Brooklyn who would't stand down from a fight'. Sure he isn't a genius, or a millionaire with a kick-ass metal suit, but the guy is equally just as amazing as any other Avenger, Coulson, and soon even Tony realized that.