I like how nerdy Bruce Banner is. It was funny how he rode in on that motorcycle and all he could say was "well, this seems horrible". And Loki mentioned how he went to many different worlds, right? Judging how he looked at the beginning of the movie, I'm guessing he met the angel named Satan from that movie titled the "Adventures of Mark Twain".
I absolutely LOVE Robert Downey Jr. As Iron man, but I also LOVE Captain America and Loki. I do adore Mark Ruffalo as Bruce Banner. I think he does an exceptional job! I could take or leave Black Widow and Hawkeye, they are not my favorite. I think that Scarlet and Jeremy do an excellent job playing their characters. Thor is okay, I think he did a lot of wrong by Loki, so it is hard to like him as a character.
Well if you mean the team themselves it's really hard to pick but I'd say Thor, Bruce and Steve are my top three but I don't hate any of the characters.