Sonic, but damn if they don't have a good reason to be.

COD. Before modern warfare 2 it was cool. Later it got toxic.

Its a tossup between the CoD and the FGC. I don't really like to single out one particular group of the FGC, but the people who insist that Melee is the best is pretty bad. They like to act like purists and it ruins any chance of any sequel to be considered good.

Sonic :/

Why is the Sonic fanbase the worst?

Has to be either Sonic or Call of Duty.

halo, gta or zelda

Honestly the answer should only be this. You haven't encountered toxic, until you talk to league players. They make Halo and Call of Duty Microphoned kids look like mature adults. 

Whats the deal with league of legends?

I think they biggest thing, anyone and just about anyone can play it. So just imagine youtube comments in video game form.