I personally like Majin Buu because he is just so unique as a villain in his first form he acted so innocent but was so deadly and sadistic(possibly the most sadistic in the series). The best part was how he evolved all the way back to his original form because he was in essence like a child growing up and growing into an evil person stage by stage and in many ways a child is the very purest form of evil because children are at very tempermental, immature, and violent and Majinn Buu was the best representation of that. Also I don't think outside the DBZ universe there's a villain like him.