Well, honestly this wasn't as easy of an answer as it once would have been. Even as I was typing this, my mind changed again. I guess I'm going to have to go with the one that really upset me. And that is…

Princess Emeraude from Magic Knight Ray earth (NOT the one from Tsubasa)

There were a couple of reasons why I chose her. First, I thought she was a very useless character - not in term of the storyline, but she was someone that couldn't do anything for herself. Also, without giving it away, I hated the way her character ending was resolved in season one. It just didn't sit well with me and gave an unpleasant feeling for the hindsight of the show.