In my view, all fandom is toxic and will always exist.
But I'll talk about a few, TF2, CS:GO, Friday Night Funkin, Minecraft, Sonic Fandom, GTA, the anime community in general and so on.
So far the Mugen community so far has been the only one that hasn't seen so much toxicity. Maybe it's the affection I have for the game that I didn't notice the flaws.