That's debatable I would think as I feel all of them played an important role in the movie, maybe Hawkeye the lest though. Iron Man I believe had the most screen time of all the team and he may be the most known of the whole team due to having two movie prior to Avengers, Hulk was probably a favorite due to the movie giving some pretty great scenes that really captured the character. Others like Black Widow also had great scenes.
 
Personally, I think Iron Man was the "best" in terms of him being the most well known and due to him having the most screen time in the movie out all of the whole team. But my favorite Avenger in the movie was Cap. :P

@War Killer: Cool

@CaptainLantern76: You should've picked Cap. By "best" I meant "favorite"

Thor.

@TheCannon: I didn't care for the on earth stuff but hiddleston alone made that movie great

I really liked Hulk in the movie honestly, to me he kind of stole the film at parts.

@Batnandez said:

@TheCannon:hiddleston alone made that movie great

Agree.

@Vortex13 said:

I really liked Hulk in the movie honestly, to me he kind of stole the film at parts.

Can you explain to me what he did that made him so good? I think he's overrated in this movie.

@TheCannon: It was more that I liked Banners character which isn't actually the hulk I guess, but he wasn't an option so I went with hulk.