Majin Buu

Majin Buu is one of the worst villains in Dragon Ball Z because he is practically unstoppable. He is horribly strong so if you can cause damage to him he will just regenerate every time, eg: One time Vegeta used a special attack to put a hole right in the middle of Majin Buu's body but he just closed up that hole and regenerated.

After that Vegeta tried sacrificing himself to make a huge blast he thought would kill Majin Buu, but it didn't, he was in 1000 pieces but he regenerated from that too.