The worst character of all time is Ezri. I hated her in 1998/1999 and still hate her every time I rewatch her. It's tragic the amount of episodes they wasted writing for her in that last season instead of, say, Garak or Dukat or Zek or pretty much anyone!
I also didn't like the crazy geniuses that Bashir discovered in Seasons 6 and 7.
Didn't care for anyone in the Mirror universe, except Sisko, who was beautifully performed with fantastic eccentricity by Avery Brooks!