Oh boy... I can already hear the sandstorms coming...
Well, I'll just get these 2 out of the way. The Undertalepants, and FNAF communities partially due to the size. Though there are certainly good sides to them. The FNAFB fan games, emphasis on last two letters, are utterly hilarious. It's like Robot Chicken got their hands on FNAF, and if you've seen Robot Chicken, you know it's good stuff. Undertale got old really fast for so many reasons I could write a small book on.
Then of course, there's Minecraft itself. To be frank this community has really slammed and stayed at the bottom for a couple years now. The creativity, friendliness, open arms, and character of the older community has really vanished. It's honestly really sad to see it change like that. Now I tend to see younger people who tend to care for nothing else but PvP. (Yes, I am completely aware that MC's sales are still steady, if not rising on other platforms.)
I won't be surprised if we see the Sonic and Halo fandom mentioned.
If I'm going to be frank I would have to the Off-Topic community is the scene that celebrates itself and it's very petty.
But in all seriousness I would add the Jojo's Bizzare Adventure, shoegaze, emo-revival, Fire Emblem, Super Smash Bros, and PC gaming fandoms as all pretty dreadful. Admittedly I do participate in several of those properties but you gotta do what you gotta do.
I've actually seen mostly good things from Undertale. Maybe I haven't poked my nose into it enough. You have the "Chara rights activists" who will pursue you eternally if you make Chara anything less than a saint, the people who freak out at you for calling Frisk he/she, and whoever created that godawful NSFW AU...
But other than that, I've seen nothing but beautiful art, great stories, and a community who's taken UT's message to heart and tried to be better, kinder people, myself included. ¯\_(ツ)_/¯
Then again, that goes for all fandoms that tend to be branded as toxic. I'm not into Steven Universe, MLP (anymore), FNAF (not really)... But besides the occasional drama explosion (and what community doesn't have that?), the worst I've seen from those fandoms is bad MS Paint art. I think people have a tendency to overblow things...
Sometimes I wonder if there's people who just type in "Undertale" into Tumblr, scroll for 10 minutes until they catch some crazy person ranting about how Frisk is nonbinary and if you call them "she" your crimes are on par with that of Stalin's, screenshot it, then post it everywhere so everyone can point at it and say, "Look, Undertale's fandom sucks."
If you go looking for trash, you're going to find it.
FNAF, Sonic, and Undertale. Not all of the people in those fanbases, just some. FNAF, from what I've seen, is a bunch of fanart of shipping and Chika boobs, which I find are cringy enough, but it may be the most tolerable of the four. Sonic, I have seen some weird ship art and it generally seems to be pretty fetishy and edgy. When I think of Undertale, the first thing that comes into my mind is Toriel inflation and not much else. I did not mention bronies, but yes, the stereotyped brony is quite cringy. Though, being on a brony forum, most people there seem to be quite great. They are very nice and I've never had a problem with anyone there. Some of them are stereotyped, but I just leave those people alone and let them do their thing.