Susan in Seinfeld. My god, re-watching the show now it's even worse than I remember it being. In a show where it was a running joke that Jerry couldn't act to save his life, she was somehow worse, as a character and an actor.
Skylar White in BB. Been covered in the thread already.
Randy in That 70s Show. They should have canceled the show at that point instead of proceeding with him.
Jess in New Girl. I don't actively watch the show but my wife does so I've seen bits and pieces. It's always amusing when the main character is the worst aspect of a given show.
There's several in LOST to choose from (mostly due to the writing) but I'll go with Kate and Charlie. In hindsight, I can't believe I made it through the entire series. So many characters are immensely stupid/dense. I tried rewatching it last winter and gave up after two episodes. Hell, I can't rewatch LOTR in peace because of Dominic Monaghan's Charlie.
The Sand Snakes in GOT. I'm pretty sure everyone hated them, no need to go into detail.
Rose in The Last Jedi. Don't get me started.
Sheldon in BBT. I only watched the first two seasons before bailing. The blonde chick isn't much better. God that show is awful all around.
Ross on Friends. I didn't watch Friends until two years ago, at the non-stop nagging of my wife, but he made it tough to get through.
Michael on The Office. Probably my most controversial pick. My wife and I both hated this show because of Michael. The character got old so quickly and annoying to the point that we had to give up on the show. He wasn't funny most of the time, only cringey and so over-the-top. It was the start of this awful trend in Hollywood/TV that's still around; that cringe = funny. You still have to be funny while being awkward or cringey. Seinfield was masterful at this; afterwards we got so many wannabes that failed miserably because all they did was make you wince, they didn't make you laugh too.