The Hip Hippos from "Animaniacs". No characters brought an episode to a screeching halt quite like they did. I'm not even exaggerating when I say that they never once made me laugh.
Oh, and Cheese on "Foster's Home for Imaginary Friends". He wasn't bad at first, because the joke was originally that Bloo couldn't put up with his stupidity. Now the joke is just that he's stupid, and he's allowed to carry entire episodes by himself. His jokes don't even necessarily consist of him saying something and another character reacting to it - now we're supposed to laugh at the setup instead of the punchline. It's like what happened to Ralph Wiggum on "The Simpsons", except it happened in two seasons instead of ten.
Oh, and Cheese on "Foster's Home for Imaginary Friends". He wasn't bad at first, because the joke was originally that Bloo couldn't put up with his stupidity. Now the joke is just that he's stupid, and he's allowed to carry entire episodes by himself. His jokes don't even necessarily consist of him saying something and another character reacting to it - now we're supposed to laugh at the setup instead of the punchline. It's like what happened to Ralph Wiggum on "The Simpsons", except it happened in two seasons instead of ten.