Black Widow

While she was the only female character of the team, Black Widow proved her worth and then some during both Iron Man 2 and the first Avengers film. A woman that can extract any kind of information from an enemy using her skills and what can be perceived as an innate weaknesses of the gender can only be a strong character.

We would have expected more of her, though, since she was actually among the first to be an Avenger, even before Iron Man.