Amen to Thor Damar! Klingons are simply stupid. "You have offended me. I fight you." They'd never get anywhere that way, always infighting. Cardassians were the closest to having a fascist vibe, too, which is a cool feel, even the way they stand.
Garak is my favorite Trek character ever, and I'd even nominate pre-Pagh Dukat as one of my top-3. When he gets into self-justifying mode, not only does he believe it, but if some of what he says is true, he actually did do some good, spare some Bajorans, while occupying them. Kira is unable to consider that. He is simply "bad" to her; this sets up one of the "good guys" (Kira) actually being morally blindered (a bit), though of course she had her reasons.
Post-ers have all commented on villain races. What about Vulcans?
I'm going to have to disagree with you on the "Fascist vibe" aspect of the Cardassians, I never though it was cool and it seems to be a common failing in Star Trek to have an entire race represent an existing ideology.
Let alone for the rather obvious reason that Fascism is deeply unappealing (to say the least).
Dukat did over see the occupation of Bajor which saw the deaths of Millions of Bajorians so any "reforms" that he undertook could not forgive his crimes as the perfect of Bajor.
As for the Vulcans I do not find them very interesting at all. Now, as for their sundered cousins...