Dukat is hands down one of the best villains ever written, not only in Trek, but in televion in general.
I agree that he was a very well-written villain, at least up through "Waltz", when he seemingly went insane and the writers made him more of a one-dimensional "evil" character. He was a swarmy b*****d and a meglomaniac and a tyrant but even I occasionally broke down and enjoyed his character. He was definitely a real "love to hate" character. Marc Alaimo did a great job with him IMHO!