I know I'm just going to sound really petty here, but the Five Nights at Freddy's community drives me insane.
Backstory time, children!
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See in 2010, Call of Duty: Black Ops was released and the Zombies mode within it started getting all focused on the story. Let's just say that the until-then quiet community of fans began madly speculating, largely due in part to YouTube stars trying to capitalise off the success of Black Ops. One picture somewhere would apparently link to a scenario hinted at in Map B, which is confirmed by a poster from World at War and if you bake that in headcanon for a day and post it on the internet you create the ultimate way to piss Evonisia off. What's worse is that Treyarch basically fucked it's own interesting story (if cliché) with the map Moon, where they just conformed to the fan's speculation knowing there would be a backlash if they didn't create an ending that would suit to them. Moon may have been an excellent map with a wonderful low-gravity mechanic, but what the fuck was the story to it?
That part I mentioned before: "One picture somewhere would apparently link to a scenario hinted at in Map B, which is confirmed by a poster from World at War and if you bake that in headcanon for a day and post it on the internet you create the ultimate way to piss Evonisia off."
It sums up the Five Nights at Freddy's community entirely. Have you tried reading the Wiki? All I wanted to know was the actions of Bonnie the Bunny and I'm assaulted by how Bonnie is really this, or the ever infamous "Foxy is a good guy" speculation. Five Nights at Freddy's has a very interesting story, I will totally grant you, but the games themselves present it horribly, which leads to this over active hive of speculation and madness that can't help but drill into my skull. The Silent Hill games knew how to do symbolism and foreshadowing through imagery, I don't know what FNaF is trying to do by comparison.
As for the best community? I actually think that the Zombies community pre-Black Ops was pretty damn awesome. It was mostly about helping each over, figuring out cool secrets in the maps or good camping specs to hide out at, whether the Ray Gun or the Wunderwaffe was more awesome (forever team Wunderwaffe senpai <3 <3). Kids weren't all that common back in World at War days, kinda like with Halo 3 weirdly enough, though Halo 3 had a very hit-and-miss community in general. The Halo Fanon community was pretty awesome as well, but I'm not sure if they count under "gaming" communities as it concerned the management of fan fiction, not the games.