[Matrixbeast]

I don't know enough Showa, unfortunately...

For Heisei, the worst villain, hands down, is Kai. Showed up during the last 5th of the series with only one line prior to alluding to him, having no real goals beyond being blatantly evil, with a lame gimmick being portrayed by an actor who clearly didn't want to be doing it.

For Neo-Heisei, as much as I want to say Medusa, I have to say that, honestly, it's Sagittarius, from Fourze. He had a plan that he didn't have a clue would work, his motivations were odd, and as a whole, his character was just dull. His followers were mindlessly motivated by him, despite having no reason to be so loyal, and his defeat, while keeping in tone with Fourze, was anticlimactic to say the least.