I'm not the biggest fan of Prince Blueblood. But if EqG counts, my least favorite character is definitely Principal Cinch. For most characters, there's at least something positive I can say about them (usually the character design if nothing else), but Cinch has an unlikable personality, one of my least favorite character designs in G4, and I don't even find her a particularly interesting villain. I guess her song was alright, but even that isn't one of the MLP songs I keep returning to.