Gul Dukat... by a widening margin now that I've been thinking about it.
Plus, he has one of the best lines in all of Trek, which I'll paraphrase: As Sisko points out the inherent moral superiority of the Fededation, Dukat nails him with "Its easy to be a saint in paradise". This is after Dukat tries to explain the far harsher material/economic realities of life in the Cardassian Empire.
[edit] And while I can't say Ardra was a great opponent, she did have one of the funniest lines in all of Trek directed at her, by Mr. Data... "Will the defendant please refrain from making opposing counsel disappear."
And for the record, I also liked Vash.