I'd like to say I thought Dukat was the most interesting villain in almost any TV show. Why? Because when he wasn't doing his job ( i.e. being evil) he actually seemed to like the characters on the other side. He realized early that Odo was a brilliant and honest detective and gave him the job that Odo kept for the remainder of the series. When a Federation craft accidentally crashed on a Cardassian planet, Dukat smoothed it over instead of making an incident out of it. He understood Bajoran culture so well that he could impersonate a Bajoran and fool Kai Winn. When he took over DS9 again with Dominion help, he left both Jake and Kira alone even though either of them could have been spying for the other side.

On the other hand, when he WAS doing his job, he could do terrible things like murder Jadzia Dax, Kai Winn, and eventually Sisko himself.