Two notable exceptions are The Soup Nazi and Newman. The Soup Nazi made FranT's list and is the unanimous number 1, so if he's the Villain King, consider the following 10 his mercenaries. I have no idea what Fran is up to with Newman, but I agree with him being in somewhat of a different category. He was never anything but Kramer's friend, and he was often in cahoots with Jerry, Elaine, and George, even if it was only ever for personal gain from every party involved.
Top 10 Seinfeld Villains10. Slippery Pete
If you don't like his name, Kramer didn't care for it either. In fact, it's one of the things they disagreed on. He's the best...and the worst. This guy just oozed "sleazeball", and couldn't even help George get the Frogger machine to safety. Bonus: he has an equally sleazy sidekick named Schlomo.
Slippery Pete & Schlomo
9. Rebecca De Mornay
Rebecca DeMornay thwarted George's attempt to return the "French Impressionist Paintings" book that he had taken to the bathroom at Brentano's, and also was the champion of the homeless when Elaine tried to pawn off the Muffin bottoms on them (just because they're homeless, they don't deserve a muffin top?). Quite the active life, a great portion of which seemingly dedicated to complicating the efforts of the Seinfeld gang.
Also, apparently this is the name of a famous actress from the '80s and '90s, and is supposed to be funny?
8. The Neo-Nazis
The disciples of O'Brien and Murphy were probably some of the more actually dangerous villains in the show, wielding guns and radical ideologies.
7. The Van Buren Boys
There's a street gang named after president Martin Van Buren, and they're just as mean as he was. They're tough cookies, but they never hassle their own kind.
6. Lloyd Braun
Lloyd Braun made George feel insignificant all through childhood, and in adulthood he continues to show George up by becoming an advisor to Mayor Dinkins then later out-selling George in computer sales before George starts lying about his numbers.
He also ominously warns George "serenity now, insanity later", which sure enough comes back to bite George when Kramer snaps and smashes all his computers.
5. Jack Klompus
Jack Klompus started as a Seinfeld family friend but turned to villain by taking back his astronaut pen from Jerry, ruining the Cadillac by driving it into the swamp (despite having ridden in one thousands of times), and leading the charge to impeach Morty from his post as President of Del Boca Vista.
As if that weren't enough, Klompus also complicates the shipment of the Morty and Kramer's raincoats from Florida to New York by not being able to get into the storage garage. All he had to do was jiggle it!
4. Kenny Bania
Bania is innocent, but to Jerry he represents all that life shouldn't be, and Jerry thinks he is a "hack" comedian. He is a constant annoyance, willing to implement his odd food-for-clothing barter system at a moment's notice. He also kills (kills Jerry!) by riding Jerry's coattails after Jerry has gotten the crowd in a good mood.
3. Sue Ellen Mishke
The heir to the Oh Henry! candy bar fortune, a.ka. the Braless Wonder, always made Elaine's life miserable by "flaunting society's conventions", but began tormenting the rest of them when she walked down the street wearing nothing BUT a bra, causing Kramer to crash his car, and Jerry to stay mum so as to not displease her, leading to a great argument between the two about their motivations in dealing with Sue Ellen.
2. Crazy Joe Devola
Joe Devola is a classic Seinfeld villain because he was developed as a villain to each of the characters somewhat independently before the story lines began to interweave in classic Seinfeld fashion. His turn as the tortured clown is legitimately scary. Better wear a motorcycle helmet at all times and have pepper spray on hand if you've somehow gotten on his bad side.
1. Tim Whatley.
Did Bryan Cranston hone his skills as a villain on Seinfeld?
He re-gifted, then he de-gifted, and then he used an upstairs invite as a springboard to a super bowl sex romp!
He converted to Judaism strictly for the jokes, offending Jerry as a comedian. Having coverage on the two major religions, if he ever got Polish citizenship, there would be no stopping him!
We have no idea what went down while Jerry was anesthetized in the dentist chair, but we do know that he went in with his shirt tucked, and woke up un-tucked.
Finally, the look he gives Jerry in the "two face" encounter is incredible.
Honorable Mention: Sally Weaver, Marselino, H&H Bagels, Gladis Mayo