Sonic fanbase hands down. There is literally NOTHING that other fanbases can do to be worse than the modern day Sonic fanbase. You can argue MLP and Invader Zim fans are just as bad. You are more than welcome to contest that they could even be worse. You would still be wrong, but you're more than welcome to try.
Invader Zim would be overlooked today, but back when it aired, it was considered ground-breaking and it would finally put Nick up on top, kicking CN off of its pedestal. It did lolrandumb humor better than damn near anything out at the moment. It was spastic, coming in without warning and ending just as quickly. The acting and tone made it even better, switching between calm dialog and elliptic fits of nonsensical yelling, with the cast acting as if it was completely normal. The art, animation, and coloring were mind-blowing for a children's show. It is probably the last truly great cartoon from Nickelodeon (sorry Avatar fans, but the finale really screwed the pooch).
Friendship is Magic was the show that girls SHOULD have been getting for three decades. The characters and their chemistry was what made the show. They all weren't a bunch of Mary-Sues or damsels in distress, nor were they all buddy-buddy with each other. While the show doesn't have the most fleshed-out cast, it serves as a very solid template of how shows aimed at an "unprofitable" demographic should be done so it can transcend negative labels. If at least that much is learned from the show by future producers, writers, directors, etc. we could have another golden age in animation within the next five or ten years.
Sonic, however has had far more bad than good at this point in time. It hasn't had a game that would be looked back upon as a "classic" since the launch of the Dreamcast. It has introduced new character after new character that none of the fans from the Genesis era liked. The games has time and time again lost sight of its original routes, and when they do try to go back to the basics, they STILL manage to fuck it up. The Sonic series has been the biggest laughingstock in the video game world for over a decade. They're like a band that ruled the world in the 80s, but haven't made even so much as a passable album since then.
Even if the fanbases of Zim and FiM are shit, the shows aren't. Sonic has NOTHING left to justify the base.