There is no character on Seinfeld who is actually likable, to be perfectly honest, but that's exactly what the show banks on. Dysfunctional adults with incomprehensible relationships clearly make for genius comedy. Take Jerry, who, being a rather flawed character himself, has the audacity to hate on other people and that too for the most ridiculous reasons.
However, Jerry doesn't have nearly as many enemies as his friends do (especially George, who makes it his life's mission to burn bridges.) The few that he does have, though, he does not like. At all.
Mike Moffit
Jerry used to be friends with Mike until Kramer accidentally spills a crucial piece of information: that he referred to Jerry as a "phony." Not one to take that lying down, the comedian openly demands an explanation from Moffit, who proceeds to stammer some nonsense about how he intended it backward, as some sort of ironic "slang."
In a later episode, Jerry and Mike continue their unspoken altercation when the latter goes and accuses Jerry of killing Elaine's fake coworker, Susie.
Katie
Katie is a wonderful character, which is basically why Jerry hires her as a manager. Unfortunately, she is absolutely the worst at doing her job correctly, regularly destroying fantastic stand-up gig options with her perennial uncertainty.
In one scenario, she casually informs Jerry that their next flight's airline pilot would be watching his show, which makes him nervous enough to ruin the whole set. Katie displaces her anger towards the pilot, which inevitably gets him thrown off the flight back to JFK (it gets worse when she promptly drives their car bang into a random person's pool.)
Three Generations Of Mandelbaum
Jerry unwittingly picks a fight with a retiree living at the same condo building as his parents in Florida, Del Boca Vista. Izzy Mandelbaum forces him into a contest of weightlifting that unsurprisingly ends with him getting itted to a hospital.
Similarly, his son, Izzy, Jr., tries to outlift Jerry (via the hospital room TV set) and is also hospitalized. If this isn't enough, Izzy's father, Izzy, Sr. tries to replicate the competition but ends up replicating the results. If Jerry hadn't found the whole thing so bizarre, he would have certainly expressed his annoyance with this ridiculous family.
The Maestro
The Maestro, who refuses to acknowledge his real name, Bob Cobb, is a source of frustration of Jerry, especially when he snootily tells him that he would never be able to rent a home in gorgeous Tuscany. Obviously Jerry needs to call Bob's bluff, so he goes on his own vendetta-fueled mission for the simple reason of making a point.
More importantly, he finds the term, The Maestro, too jarring to use in regular conversation, which creates another layer of friction between the two characters, for instance, when he begs Elaine to call him "Jerry the Great" as a form of overcompensation.
Tim Whatley
Nobody is more abhorrent than a regifter, at least in the opinions of Jerry Seinfeld and Elaine Benes. When their dentist friend, Tim Whatley, presents Jerry with a fancy label-maker as thanks for his Super Bowl tickets, Elaine reveals that it was the same gift of gratitude that she had offered him recently.
He is never sure of Tim's intentions, given that he suspects foul play occurring while he was under the influence of nitrous oxide. Is Jerry Seinfeld an "anti-dentite"? One may never know.
Poppie
Poppie starts off as a benign restaurateur (and the father of Jerry's then-girlfriend), but their relationship soon changes course. First, the fact that the chef doesn't wash his hands after going to the toilet results in Poppie's being closed under the guidelines of the NYC department of health.
In "The Couch", Jerry impishly creates tension between Elaine and Poppie by bringing up the contentious matter of abortion, but this backfires most horribly when the course of events leads to the old man urinating on Jerry's new sofa. Kramer doesn't seem to care about this at all, though. Maybe he should have been on this list, too.
Joe Davola
Jerry's understandable hatred of Joe Davola only began when the latter began leaving him rather ominous threats (while one of his best friends was off on a whirlwind courtship with the wannabe Pagliacci.)
His mere presence is terrifying to poor Jerry, who spends considerable amounts of time hiding out in safe spaces and waiting for cops to keep their promises. Joe is last seen screaming and jumping out of the audience during Jerry and George's NBC pilot, a scene reminiscent of John Wilkes Booth's assassination of Lincoln.
Uncle Leo
Jerry tries to hide his dislike of Uncle Leo for his parents' sake, but the old man sees through the paper-thin facade on several occasions. His general aura is one of enthusiastic friendship, but the regularity of his catchphrase (and the constant boasting on behalf of his invisible son), forces Seinfeld to avoid him as much as possible.
And this is excluding the worst fact: that Leo is a snitch, always exposing his nephew's various indiscretions to his parents, something that never fails to get Jerry into more hot water.
Kenny Bania
Bania is like George, only infinitely worse (imagine that if possible.) Jerry really abhors this character, simultaneously deeming him the most boring and the most annoying person he has ever met. All Kenny wants is for his friend to buy him dinner, which Seinfeld is coerced into because Armani until he postpones the meal by just "having soup."
When Kramer sells Bania a vintage suit containing Jerry's fur-storage receipt, it's the latter who has to take him to dinner (again) just to retrieve his own possession. In one satisfying scene, Jerry refuses to let him cut in line for the Soup Nazi's delectable dishes, smugly claiming that he wouldn't want to get himself banned for doing so.
Newman
Jerry's hatred of Newman cannot be resolved without divine intervention, such is the extent of his emotion. In fact, the character was originally created to be the "Lex Luthor to Jerry's Superman", a role he fulfills with gleeful villainy.
Newman takes great pleasure in making his neighbor as miserable as he possibly can — whether it's telling Helen and Morty about their son necking during Schindler's List (1993), gathering forensic evidence for Jerry's barber, or simply spreading fleas throughout the entire apartment building. But then again, what would Seinfeld be without Newman?