I would just to add my thoughts on this interesting topic (which i somehow missed!) Sorry in advance for the lack of names, my memory isn't that good!
> The main bad guy from Chariot of Gold I thought was a good villian, in the sense that he was clearly mad in the way he was thinking.
> The bad guy who knocks out Michael in Knightmares I thought was a good viliian. Just made you think why hadn't it happened before, with any explosions that Michael had put up with!
> Cameron Zachary. The guy who ordered Michael's "death" to begin with. In some ways he is more evil that Garthe!
> The main bad guy in Knight In Disgrace. Although he isn't the meanest or evilest, he was a lot of power and wealth with the use of technology, which makes him dangerous.
> Next, i would say Gordon Baxter aka Chameleon. He may not be like Garthe or Tanya, but he could evade both Michael and KITT, and not many villians could do that more than once!
> The driver from custom made killer was one of the baddest. He managed to beat up michael pretty bad, even more so than Garthe did when he went into Kitt.
There's just some of my thoughts.

If i had to pick one, it would be Cameron Zachary without hesitation.