I know I am going to get hated for this but in my opinion, I cannot STAND MLP. It just seems like an immature fanbase with most of it being composed of 10 and 12 year olds (and for 30 year old guys who are into it, they are way too old to be into that kind of thing).
More an example of a hatedom, but Anti-Bronies and Anti-Furrieare often very obnoxious. About 95% of anti-bronies and anti-furries that I have met rely solely on stereotypes of the fandoms (such as that it is a "fact" that bronies and furries tend to be perverts or creeps).
On the fanbase side, Steven Universe is the worst fandom ever. The entire Tumblr community once drove a girl to suicide for drawing some characters too skinny, they overreact to slight color differences in fanart, and I've stumbled upon a fair amount of Rule 34 as well.
-Minecraft. As you can probably tell, I love Minecraft (and have been playing it for well over 3 years). However, there are some archetypes of Minecraft players that annoy me a lot.
A: The 8-year-old kid who says "MINECRAFT RULZ111!" and doesn't listen to anyone's opinion that could be critical (in that the game may not be perfect after all...)
B: The people who complain about how the game was "so much better back in Alpha/Beta" and constantly rant about new updates. (IIRC, the Minecraft launcher allows you to downgrade back to those versions now)
The haters can be pretty nasty as well, but I do sympathize with them when they are irritated by the fanboys. (The haters can also say they don't like Minecraft because of the age-old reason "The graphics suck.")
-One Direction. I've never had any use for One Direction's music and consider them to be a mediocre boy band, but their fans especially make me hate them. I've actually read up on their fandom, and some of the rules the "Directioners" set up actually feels like a cult to me.
Oh yeah, and then there was that whole thing that happened when Zayn left...*facepalm*
-Justin Bieber. Similar to One Direction, minus Zayn's leaving, but plus for that I find Bieber to be completely talentless.
-Bronies. I don't hate MLP (I don't like it either though), but they have irritated me in the past. (And I'm still a bit confused why grown men would watch a show targeted towards little girls.)
-Five Nights at Freddy's. Similar to Minecraft, I like the FNAF games. However, it does seem like most of the FNAF fandom is composed of Archetype A that is listed for Minecraft fans (as well as people who like to draw creepy fan art)
-SkyDoesMinecraft. For similar reasons as Minecraft and FNAF (I actually think his videos are funny, but his fandom can be quite annoying, and the butter thing was pretty ridiculous)
-Apple fanboys. Like Minecraft Archetype A, but perhaps worse. No phone out there is perfect (trust me, it's going to screw up sometime) but some believe that iPhones are supposedly God's gift to the world. (despite the fact that iPhones are very expensive and I find Apple's support system to be sub-par) And, they also criticize people who have different phones than them (I have an Android).
To be fair though, a lot of fandoms are centered around pretty cool things, such as Minecraft (there's a lot of creativity there) but they're unfortunately dragged down by the vocal minority that really ruins the look of the fandom itself. (haters can also be just as bad)
-Roblox (never had any experience with Roblox, but hear the fanbase is terrible)
-Sonic (same as Roblox)
-Fundamentalists (mostly Westboro Baptist)
-Feminists (I actually don't hate feminism, but "feminists" like Sarkeesian really irritate me)
-Taylor Swift's fandom (like One Direction, but not nearly as mediocre music and not as strict)
-And finally, the 12 year old "MLG SWAG KIDS". Swag is stupid, sorry to say that and no one cares how "pro" or "boss" you are. These overlap with PewDiePie, SkyDoesMinecraft, FNAF, and at least half of the rest.
Honestly, this post is gonna be kinda controversial...
Never go near a Roblox fanboy with a 39.5 foot pole. You heard right about the fanbase. Scream whenever Minecraft is mentioned, filled with 6 year olds, claim that anyone better than them is a hacker, cram innuendos and toilet humor into the forums, and the site has become a breeding ground for trolls and online daters.
Creepypasta fans, ****in' hell. They literally STOLE two Marble Hornets characters and ****ed them up, also the cheesecake joke started out as a fat joke against Tim Sutton, the actor for Tim Wright/Masky