I’d always restrict this debate to solidly defined characters. Ones with more than a few minutes of airtime. When your role in the writing is to be an antagonist for a children’s show, you often can’t be shown as having too many good qualities. I can’t be made to believe that Diamond Tiara and Silver Spoon are pure evil as they are presented in the show. (Although, Babs Seed was able to be given good and bad qualities, but the episode also dictated that.)
I don’t like Sapphire Shores, but I could say that she’s only played one role in the series so far. As she was presented, she needed
to be diva-like so the audience could know she was a diva. I’d wager that Sapphire Shores is a much less “My water isn’t room temperature! You’re fired!” sort of gal and more “Yes, I do have that effect on ponies” in the same way that Rarity is “Yes, I am quite glamorous, am I not?”
But to answer the question, I might go with Rainbow Dash. She’s been shown to have really good qualities, but at the same time, she’s had moments where I just hated her. I can’t think of any other characters that have invoked that sort of dislike for me.