For me it's a tie between Gilda and Diamond Tiara. I just don't like both of them so much.
First off, Gilda was a jerk to Fluttershy. And even though Fluttershy is not my favorite pony, she is still a sweet pony. She is so kind, yet Gilda is so mean to her.
Second of all, Diamond Tiara is also a jerk to Applebloom, Scootaloo, and Sweetie Belle. And all she ever does is make fun of the three of them for not having their cutie marks. And without Diamond Tiara, Sliver Spoon wouldn't have been so mean to the CMC's.
It's hard to decide because I don't like either of them. It's a really hard decision for me. But it has to be one of the two of them.