Shonen anime villains make for some of the most fascinating figures in their genre. They generate the challenges that the heroes must endure, they create a sense of danger and suspense, and they reveal the dark underbellies of seemingly peaceful anime worlds.
But which of these villains rank among the best of the best? With so many great options, it's hard to narrow it down to just a few. Some are great because they have deep, sympathetic backstories that perfectly explain their motivations and make it difficult to hate them, like Ultear Milkovich from Fairy Tail. Some, like Frieza from Dragon Ball Z, are irredeemably evil monsters who you just can't look away away from, no matter how much you might want to. Villains who force the heroes to rethink their world, like Pain from Naruto, are fascinating too.
Every great shonen villain brings something different to the table. The one thing that they all have in common is that they're indispensable when telling a great story.
Hisoka has one goal, and one goal only: to enjoy himself through battle as much as possible. Morals are completely irrelevant to him, which means he's just as likely to help out the good guys as he is to oppose them. That's what makes him such a great villain – he's impossible to predict, and fascinating to watch.
Sometimes, the best villains are the ones who take you by surprise. Sōsuke Aizen pretended to be a loyal member of Soul Society, but he was actually plotting its downfall the whole time. When he reveals his true evil face it's a total shock, but it shows exactly how devious (and therefore delightful) Aizen is.
Meruem - 'Hunter x Hunter'
Meruem starts out so vicious that he literally kills and eats his own kind (chimera ants) if they fail to show him total deference. He sees himself as nearly flawless, and his incredible strength backs up his viewpoint. That arrogance and cruelty begins to dissipate when he meets Komugi, a young blind girl who he forms a bond with through playing a game called Gungi. This unlikely connection serves to deepen Meruem's character, and makes him a much more interesting villain.
The Japanese government literally set Makoto Shishio on fire because they thought he was too dangerous to be kept alive, but that only made him more dangerous. He decided to take over Japan, gathered terrifyingly strong allies to help him do that, and got to work.
He's a formidable opponent for Kenshin and his other enemies, adding genuine tension to the show that might not be there otherwise.