Whether she's played by Alice Krige or Susanna Thompson, the Borg Queen, hands-down. Rarely do I so vehemently hate a character not because of who they are in context of a story, but for how they poison a narrative. She was idiotic in FC, and FC (and the concept of the Borg) were still good despite her IMO. Partially because I got to believe that Picard snapping her spine meant she went bye-bye for good. Scorpion seemed to support that. Oh, I wish.
Dishonorable mention goes to Diana Muldaur as Katherine Pulaski (show always listed her as a guest star, not a main, so). I always liked Crusher, so Pulaski was an unwelcome change in S2 already. She came off as really crotchety and had this pissy attitude about Data that got very old. She wasn't awesome like McCoy about it. She came off as just being a bitch.
Well, even with at his most racist McCoy at least had the propriety not to question Spock's actual personhood...

It wasn't that I minded seeing a character espouse these hateful views--conflict is good--but she was permitted to go
way too far because Data lacked a capacity for self-defense in that regard and no one stepped up to tell her to just stuff it.
I'll agree with John de Lancie. Not his fault, but Q sucked post All Good Things. Even "Death Wish," which could only have been salvaged if Charles Bronson had actually played one of the Q. Q should never have been involved in Voyager--or DS9. Or QPid. WTF was up with that?