This isn't a 'Which artist do you hate the most' or 'which stanbase do you hate the most' (although chances are it will become that) - which stanbase is the most UNPLEASANT to be around and state WHY.
My vote goes to the Ciara stans. See: Any Ciara thread.
Monsters. Some of them are just so obsessed with their fave, that they can't see different perspectives. The Nicki stans besides Willy., Reed and a few others are becoming increasingly intolerable though too.
I voted for Beyoncé because they seem to have lots of threads with the sole purpose to stan and Gaga because they vote for her on every poll even if it doesn't even make sense.