I'll throw my two cents in and go with Sugar Belle. Her relationship with Big Mac was totally rushed and out of nowhere. Even if we ignore Cheerilee and Marble (I'm not going to get into debating who was a better choice because it's beside the point), Sugar Belle is an extremely one dimensional character with zero personality, her only characteristic is being "nice", was this really the best choice for Big Mac?
This doesn't even get better when they start dating. You can't make me care about a relationship that didn't even have a basis to begin with. The episodes that deal with them are extremely boring, it's like the writers knew their relationship got off to a weak start and tried to fix it with supposed development, but it just doesn't work. It's like having a poorly proportioned drawing, no matter how much you give it an amazing colouring, it will still look disproportionate, to some it might go unnoticed, but to those who know something about art it won't.