I guess there are loads really.
Some people were completely annoying in every sense of the word, but the moment they're banned or leave, someone new comes along, who is just as bad or worse.
I could say Trainman1, since he was really annoying.
Although, GreenBH was exceedingly annoying and even moreso once the whole "Tyler66- Pokeball" thing came up. It was at that moment as well though, that Tyler66 became a nuisance and an idiot, since they were both just trying to spite each other.
Those two were a completely horrible bunch, who would piss you off in almost every topic they posted in. However, in comparison, they never created a large amount of topics in every Forum possible, full of spam. In this case, Chesse was the most annoying.
Simply put, they're all as bad as each other, but all for different reasons. Whether it's just plain stupidity, being completely arrogant, or spamming.
I don't think you can safely say there is a most hated person, just a lot of people who are all equally hated. :/