Hawkeye . . . I was influenced by a friend of mine. XD His history could've been expounded on a lot more. All you know about him is what Black Widow's given. But besides that, there's nothing to work on.
Though originally, my favourites were Stark and Rogers.
Loki is soooo amazing. I'm going to see this movie just because of him. I mean, I like comics in general...but daaamn, did they get an awesome actor for Loki.
Out of the Avengers though...I guess Iron Man. Those movies were cool and funny. I might pick Black Widow, except I haven't seen Avengers yet and can't form a strong opinion on her. I don't know if she was just thrown in for eye candy for the guys, or if she actually is awesome.