The FNAF fanbase.
The MLP fanbase.
The Otherkin fanbase.
The Furry fanbase.

Personally, all four.
And those who live under a solid rock of granite, don't come into this thread and go all "HURR DUHRR, WHAT IS A FURRY?". Kthxbye.

Anime. Insult some poor fool's "waifu", or even do so much as slightly criticize it, and you'll never hear the end.
On the bright side, it's a fun place to flamebait.
Same goes for the lesser-known but equally-insane kpop fans. Except you have actual Asians with those rather than just lonely overweight dues, so watch it if you're gonna try real-life baiting. I recommend you do it from afar.

THE TOAD ELEVATING MOMENT

We have with us in the studio today a man who makes pointless threads looking for a flamewar. Mr. Xel.naga, how long have you been interested in making such silly threads?

I have no idea what this is, but can I join in?

Sure you can. Just educate yourself first.

Oh! I remember this sketch, and feel horrible for forgetting about it.

I'd say the "Steven Universe" (SU) Fanbase personally, when a fanbase turns on itself so hard that it drives one members to attempt suicide I wouldn't call it healthy.

It's sad & ironic that the SU show is about inclusion yet the fanbase has some sharply "exclusionary" elements in it.

The one on my desk. Stupid fan keeps falling over because the base is broken.

Even the GOG fanbase?
Even the PAJAMA SAM fanbase!?!?

We have our zealots that bring GOG a bad name. But they're not nearly as bad as the Linux guys. They're more toxic than Call of Duty kids.

The growing fanbase for the most flavor of the month indie game to date: Undertale.

I expect so SO many MatPatt's "Theory" Videos at these point :P

True. That said I definitely want to see that game show up here - http://www.gog.com/wishlist/games/undertale. (It is DRM-free on Humble Store, though.)

Fanbase that can actually change the law or make people lose their job. Rather then just talking and annoying other people.

I wouldn't trust any fanbase with my money. That is just how it goes on the internet.