It took me a while to decide, but I think Diamond Tiara. I've decided Trixie at first, because she actually hurt other ponies physically, (EX: Rarity's hair turning green; wrapping Applejack's hooves around by a rope; creating a whirlwind to attack Rainbow Dash), but then again, I just remembered that Diamond Tiara forced the Cutie Mark Crusaders to keep making those magazines, and even though the three have felt guilty, Tiara STILL forced them to continue making the papers, because they were "funny", not caring whether they were hurting other ponies' feelings or not. That's pretty evil. Yes, Trixie may have been a show-off, she may have bragged about her powers in front of other ponies, but Diamond Tiara embarrasses Apple Bloom in front of her class about her cutie mark, brags about being special, tries to embarrass her at the party once, even made fun of Granny Smith and called her an 'ugly coot' (if that's spelled correctly). Dang. I'd have to say Diamond Tiara, really. Trixie may have done the same things, but...probably not force other ponies to hurt others' feelings.