Probably the Roosterteeth community. Their content pretty much defined almost a decade of my life but the community was composed of some of the most toxic people I've ever encountered online.
Also, it's not a fandom but I used to browse an unhealthy amount of 4chan up to ~2017. Very strange crowd there
I need to catch up on it! I'm like months behind at this point.
A lot of the old groups are still making content. Shoutouts to Bruce Greene and Lawrence (and KassemG) from Funhaus/Inside Gaming are doing a letsplay channel called BroughtYouThisThing, it's probably the only reason I get on YouTube at all anymore.