The original anime flame war: "Anime is just Japanese for "cartoons" vs. "Anime is a special artform all its own and utterly and completely different from Western animation".
There is an ever present opinion that though you may draw in the style of manga, though your comics may read from right to left, you can never call your comic a "manga" unless you are Japanese but if you make an animation in that style or any other you can call it anime because to the Japanese it's always named anime.
Asking "Why is Naruto such a crap anime" on any anime forum gets 3 different responses; non-fans saying it is, fans saying it isn't and a 3rd group that actually look at the question and point out its foibles and short comings, and try to answer it logically and factually. The 1st and 2nd group will fight, and the 3rd inevitably joins in, and can take either side.
Just try pairing any two characters together: 90 percent of the time, you'll get Ship-to-Ship Combat.
Discussing One Piece vs. Naruto (and sometimes Dragon Ball, Bleach, and lately Fairy Tail added in) has been a legendary conflict for fans of either series for the past 14 or so years they've been running. It's a battle of what "character type" the fans like most, One Piece being pirates, Naruto being ninjas, Dragon Ball more-or-less being aliens, Bleach being samurai, and Fairy Tail being mages(kind of).
One of the most offensive heresies you can use towards an anime is calling a 4Kidsdub of one good.
Even implying that any Gundam is better than another will spark a flame war that can and most likely will span hundreds of pages, the most likely outcome of these flame wars will either be a Mod locking the thread, or the comments section of a video being closed. Though there have been times where there is a peacefulresolution in which almost every side will come to an agreement and end up having a pleasant conversation while any ragers will continue arguing amongst themselves.
For the UC fandom, any mention of "who was right" in the One Year War will cause a major flame war.
There are similar wars about the different dubs for the Miyazaki classics Castle in the Sky, My Neighbor Totoro, Kiki's Delivery Service, and Porco Rosso. All four had previous dubs distributed by Streamline. While Carl Macek handled the older dubs of Totoro and Kiki, Laputa and Porco's older dubs were produced by unknown studios. Macek wanted to make better results for Miyazaki because he thought the '80s dub of Laputa was rushed, "phone-booked", and not very well done. Of course Disney redubbed all four movies, but be careful about praising any of them, particularly Laputa and Totoro to fans of their older counterparts; they will go on and on about how "superior" and "perfect" they are to their "horrible, butchering" Disney counterparts. Even on videos containing footage from said dubs. And then there are Disney dub-fans defending them.
Like in the Comics section, there is the "Can X beat Y" arguments. The most common characters to be matched up with another are Goku, Naruto, Monkey D. Luffy, Ichigo Kurosaki, and the most recent addition is Saitama. Simply implying that X can beat Y will start arguments worse than if the two characters actually fought.