[Sentai]

I'm with Brainy. The Psycho Rangers. Including Pink from Lost Galaxy, they lasted two seasons. And were perfect as villians. They had a clear motive, they were awesomely VO'd, and the best Evil Rangers we'd ever gotten in a series.

Other than that I'd go back to the original seasons. Phirantishead (Who's name I just butchered). And his boss. Lord Zedd. He wasn't a monster of the week, I know. But he was the first original American Made villian. And I hate they dumbed him down after his first appearance because of complaining parents. When he turned around on his throne, Python that became his staff, all menacing.. Goosebumps.
But if we're doing true MOTW I'd have to go back even further. Lokar. The all-powerful giant floating head in the sky with his Mutitis creature. I loved him so much. Especially when he paired with the awe-inspiring Cyclopsis.

God those original couple seasons had awesome villians.