Bizarro: All the powers of Superman but none of the Morality.
The White Martians: they can literally be anyone and telepathy is the scariest superpower
Bizzaro is both scary, and yet sad. He's all of Superman's power with none of his judicious restraint. He''s basically a tragically powerful Lennie Small.
That doesn't make him one bit less terrifying, just heartbreakingly melancholy. Which is why he's one of my five fave Superman adversaries.
Shouldn't yellow space bug Parallax literally be the scariest? That's his whole thing...
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Whatever this thing is....it's like the artist didn't finish figuring this thing out before drawing it.
Mr. Mind: an alien that can crawl into your brain and control you.
Brainiac can be creepy as hell, and truly dangerous with his mind powers. He's like an ancient space vampire.
Doomsday, because he's unstoppable and cruel.
Parasite. The name says it all. He can be really visually creepy and can absorve the power of even Supes.
That is the best description I've seen of Bizarro, he's a tragic villain, who in the wrong hands (Lex) can be manipulated to do bad things, but in the right hands (Superman) can do good ones. He reminds me of Frankenstein.
Mr.mxyzptlk.
I listened to carl sagan on youtube once explain how a 4th dimensional being would interact with us. Objectively its pretty terrifying. This guys a 5th dimensional.
Whatever he is really... it isn't a frumply little man in a bowler hat.
My priority is enjoying and supporting stories of timeless heroism and conflict.
Everything else is irrelevant.
I think there's a difference between things that make you feel like you're going to upchuck and things that are pyschologically disturbing. I don't like how some artists have transformed the Joker into what looks like the villain in a slasher movie. I think it's more interesting to play on the contradictions in the Joker, where he looks happy and benign and that disguises the malevolence inside.
Back in the 1970s, the Spook was one of the spookiest villains. He was one of the many inscrutable characters that DC had a penchant for. You didn't know if he was alive or dead, if he was using trickery or if he was supernatural for real. And the comics refused to give any answer. Which was always unnerving. I understand that since then, DC has given away all his secrets and explained the character and made him mundane. Which just tells me that they didn't understand the great attraction of the villain. It was not knowing for sure what was up with the Spook that made him such a threat.
It's like explaining the Time Trapper--why would you? The Time Trapper was always much more threatening when we didn't know what he was or why he did the things he did.
Back in the 1960s, Gardner Fox wrote a bunch of interesting one issue villains. One of my favourite villains of all time was Pete Maddox in the "Frigid Finger of Fate," DETECTIVE COMICS 375 (May 1968). The issue had a cover showing Maddox intending to assassinate Batman and Robin in their open Batmobile, in a scene that recalled the assassination of JFK--so it was particularly chilling, not just because of the President's assassination, but also in that year of 1968 MLK had just been assassinated before I got this comic and RFK would be assassinated only a few weeks later.
The story tells how Maddox has precognitve dreams, but to have these dreams he has to be exceptionally cold. The whole story is about Maddox--Bruce, Dick and Barbara are really just background characters. We're in the mind of this guy--who doesn't start out to be such a villain and, because we see the world from his perspective, you almost want him to come out on top. But his "gift" of second sight takes him down a terrifying path with no good outcome. That story always gave me the shivers.
Jane Doe. She skins people she's killed, and then wears their skin like other people wear clothes.
Professor Pyg. He doesn't necessarily kill people, but what he does is worse. Kidnaps people of all ages, and then breaks their minds with drugs, torture, and lobotomies. From there, he attaches a mask to their face, permanently, and brainwashes them into being a willing dollotron. Doing his bidding without hesitation.