One-on-one fights can be fun, but what if you don't want a whole episode or scene dedicated to just one person showing off? That's how you get Goku doing the same Kamehameha for three episodes. Sometimes what you want isn't a tight focus on a couple of characters, it's a melee that brings in a full cast. Another thing that great anime fight scenes need are high stakes. While a fight to the death isn't the only way to make viewers care about the outcome, it's a pretty effective method.
Sometimes the most exciting thing is a straight up battle royale in which a huge group of characters get in on the fight. This can occur in a single arc or even an episode, as it does in Naruto and Kino's Journey, but it can also take up the entirety of the plot, as it does in Future Diary and Deadman Wonderland.
Which are the best battle royale anime? That's up to you.
Not every part of Hunter x Hunter focuses on a battle royale, but the fourth stage of the Hunter Exam serves as a thrilling exception. Gon and the other participants are let loose on an island, where their job is to take possession of each other's badges. This wouldn't necessarily be a death match, except for the presence of Hisoka, a kill-happy clown who uses the competition as an opportunity to lay waste to as many people as he possibly can. What keeps Gon alive is less his battle skills than his strategic abilities, but it's still pretty gratifying to watch him come out on top with the help of his trusty fishing pole.
Though most of the fights in Naruto are life-or-death, few of them drag in so many people at once. But during the second stage of the Chunin Exams, a massive battles ensues, with unexpected enemies appearing on the scene and creating chaos. The protagonists will have to overcome a team of powerful ninja from the Village of Sound, Gaara before he decided that killing people was a bad idea, and worst of all, Orochimaru's attempt to kidnap Sasuke for nefarious and disgusting purposes. What was meant to be a dangerous competition intended to test their skills ends in a life-or-death skirmish that the ninja-in-training barely survive - but which prepares them all the better for further battles to come.
In the beginning of the Dressrosa Arc in One Piece, we are introduced to the Corrida Colosseum, where four intense last man standing battle royales are fought to determine who will move on to the next round. The combat area is surrounded by water infested with 'fighting fish' that want nothing more than to destroy you. It may only be one arc in the series, but if you love battle royal moments, this will surely scartch that itch for you.
Each version of the Fate series differs in its content and themes, but one thing they most have in common is the epic showdown that is the Holy Grail War. Whoever gets their hands on the grail can have any wish granted, but getting there will require doing battle with a plethora of incredible weapons, including one sword so powerful that they literally created the earth. Plenty of blood will be shed in pursuit of the grail, but who will emerge victorious?