Spoiler for like, any book existing.
I think mine has to be the villain in Mirror Man. Idk, he wasn’t intimidating, he wasn’t interesting. He was meh.
Also the villains from The Chain. Dear God they weren’t sympathetic in the slightest. The book tries to act like they were atleast pitiful but FROM THEIR INCEPTION, they were just evil assholes who enjoyed causing pain. The book itself wasn’t all that great anyways.
Arguably a villain protagonist, but Frank Chalmers from Red Mars always gets under my skin. He’s a coward, awful, asshole murderer who basically jeopardizes the colonization of Mars and the revolution for a couple bucks. Hate that guy.
I’m blanking on most villains though, going over the books I’ve read in the past couple of years, none has a villain that really stands out. War and Peace has no villain except maybe Napoleon and smaller, localized villains; Frankenstein has both Frankenstein and the Creature but I wouldn’t call either "the most evil villain ever; Piranesi has villains but they aren’t that evil…
I think the most evil one would probably be, idk, Hitler in Slaughterhouse 5 or Paul Atreides, or any of these mass murderers.
I’d put Sauron there though because of the building of a slave army hellbent in being evil.