Joffrey Baratheon. Ramsay Bolton. Olly.
These are some of the most evil, despicable characters inhabiting Westeros, rightfully despised for their actions, and are the worst characters on the show in that sense.
But what about the characters who don't necessarily do bad things, but are just bad characters?
Two of Game of Thrones biggest strengths are its cast and characters; they usually get the right actor for the part, and even the minor players fit the world and contribute to the story.
Occasionally, though, they do get it really wrong. This isn't focused on the many random smaller characters who didn't really contribute much (like, say, Mero), but those with at least a decent role in proceedings, who were underwritten, badly acted, really irritating, or just plain boring.
Thankfully everyone in Westeros gets their comeuppance eventually (and many of these already have), but whether they're long dead, still living, or a combination of the two, they represent the worst the show has to offer.