The Simpsons. Anything about The Simpsons. We're not even in the golden age of Simpsons flamewars anymore. The phrase "Worst Episode Ever" came from the most common fan reactions to every single Simpsons episode on old Usenet forums in the '90s. In fact, Simpsons flamewars and Fan Wank on Usenet were partially responsible for the rise of the internet.
Traditional animation vs CG. Some of the arguments in the Tangled forums before the movie was released made the title change controversy look forgivable.
The "TRUKK NOT MUNKY" mentioned in the page description above comes from just one of the incessant Transformers flame wars. Every single fandom can start a flame war between fans. Beast Wars and Beast Machines are the worst offenders (thus where the saying comes from), followed up by the 2007 movie.
Most fans would argue that G1 is better than every other season ever made.
There's also the matter of whether Memetic Bystander Derpy Hooves' speaking appearance in "The Last Roundup" was an offensive portrayal of the mentally handicapped, or whether Moral Guardians vastly overreacted to the fact that a character was portrayed as The Ditz. Then the episode was edited to remove the Unfortunate Implications, and a whole new series of flame wars started over whether the changes were justified.
Although it seems to show up less, people will fight over whether or not Avatar: The Last Airbender qualifies as anime or not. (No, it does not matter that the creators said it wasn't.) And that's all we'll say about that.
Danny Phantom and its shipping wars. If you even just mention the characters Sam and Valerie in one sentence, you will inevitably unleash chaos in the form of 'x sucks and y belongs with Danny!'.