I haven't seen the movie yet, but I'll go ahead and say Tony Stark/Ironman. I think I like Robert Fowney Jr as Ironman better than the actual comic book Tony Stark.
In terms of the actual Avengers comic. My favorite is Janet van Dyne/The Wasp, she named the group after all! With Hank Pym/Ant-Man,Yellow Jacket,Giant Man as a close second. Iron man would be third.
I never liked Captain America, I guess because I don't like patriotism. The Hulk I always found so boring as a child, but as I get older I start to like him a bit more, and Thor is pretty cool I guess, but I was never a huge fan, not sure if it was because he was a god or because he personlity was pretty boring *In the comics*. I tend not to like over powered super heros, like Thor and Superman, but they are alright I guess.
I also love Spider-man, but he doesn't join the Avengers until New Avengers.