Slimer, Orko, Uni, Snarf, Scrappy Doo or some other annoying little critter?
Of the ones I’ve watched, Scrappy wins (loses?) by a very great amount.
I’ll add Wendy and Marvin from the Superfriends as contenders for second place. Worse than the Wonder Twins from the same show that might also score a top 10 place.
Godzooky.
The Great Gazoo?
As a Canadian, I’ll offer up Newton the centaur from The Mighty Hercules.
I didn’t like Hardy Har Har, who was Lippy The Lion’s sidekick. All he did was whine and foretell failure and disaster.
Patrick Star.
This was my answer.
Though Orko from He-man is up there.
Talk about a weird stutter!
He even has his a Trope in his name!
Bat-Mite. Makes you want to go to the bat-hroom and puke.
Uni, in that it has almost all of the traditional anyone cartoon sidekick tropes (“cute”, constantly gets into trouble, needs rescue but somehow saves the day for “reasons” just often enough to evoke sympathy) wins the contest by a nose because of the never-to-be-sufficiently-muted bleat that grates on my ears every time they’re on-screen.
I hear unicorn steaks are tasty, that may be a plus in their f(l)avor though! 
What cartoon is Uni from?
From one of my childhood favorites, Dungeons & Dragons:
(Uni was cited in the OP, sorry, otherwise I’d have made the mention)
I never realized the D&D Cartoon was an Isekai, that is kind of funny. I knew the cartoon existed but never watched it despite playing D&D since 1977. I was a little old for the show when it came out.
I was perfectly timed for it, I was 8-9 when it started, and had just started playing Dungeons and Dragons in the big Color Coded Box sets (the famous Red Box Basic edition that came out in 1983). Out of nostalgia, I actually own one of the sets of boxed DVDs for the TV series. There are only individual episodes I can bear to watch however.
Seriously, other than one other cartoon (Thundarr the Barbarian) it was the only cartoons that totally captured my interest that weren’t re-cut, translated or otherwise maimed Japanese anime (Tranzor Z, Starblazers and Robotech being my other favorites of my childhood).
Bandit from of Jonny Quest, so out of place in a great action series.
Prickle and Goo. Pokey is everything Gumby needs for a sidekick.
Spritle and and his pet chimp Chim-Chim from Speed Racer.
BTW, Trixie was hot.
…and she could fly a helicopter.