Kid Buu

Main article: Buu

Kid Buu is the original form of Majin Buu and is also the final villain in Dragonball Z. He transformed from Super Buu when Vegeta tore down Majin Buu from his head. The original Majin Buu had his appearance before absorbing the previous South Kai and Dai Kaio, and he is a mindless killing machine. With the purity of the kais influence inside of him gone, he is much stronger and is a pure, unrestrained evil.

The Anime
After becoming Kid Buu, his first act was to blow up the earth with a giant pink energy ball, killing Gohan, Goten, Trunks, Piccolo, Tien, Chiaotzu, Korin, Yajirobe, and (though it is also possible that Super Buu killed him with his Human Extinction Attack first, depending on whether or not he evaded it) Android 17. He then used Instant Transmission to destroy planets until he arrived at Grand Kai's planet. He fought Krillin and Yamcha, who were already dead, and nearly blew up the planet before Full power Super Saiyans Goku and Vegeta powered up to draw him over to Supreme Kai's planet.

Through rock-paper-scissors, they decided Goku should fight him. Goku first fought Kid Buu in his Super Saiyan 2 state and later as a Super Saiyan 3 evenly matching with Kid Buu, but began losing his energy due to not having mastered the Super Saiyan 3 state and his earlier battles against Buu, Vegeta fought Kid Buu as a distraction, even though he knew that if he died while already dead, he would not exist anymore and could never be revived again. Vegeta was at a disadvantage and Buu was about to end his existence when Hercule, believing himself to be in a dream, appeared and insulted Buu. Buu tried to attack him, but was unable to because he was getting a headache caused by the original Majin Buu, whom Kid Buu spit out like a piece of chewing gum.

Majin Buu then fought Kid Buu, but Kid Buu had all of the advantage and beat up Majin Buu, with Hercule attacking him (and not doing any damage). When Goku lost all his Super Saiyan 3 energy, Vegeta devised a plan that involved using the Dragonballs to revive Earth and all its inhabitants (Vegeta and Elder Kai returned as well). Goku then began gathering energy for a Spirit Bomb, but no one on Earth (aside from other Z Fighters and their friends and families) would give up energy. When Majin Buu was clearly losing energy and close to death, Vegeta re-entered the battle, and Goku asked people to give up energy. A few of them did, but everyone else thought he was trying to steal their souls - until Hercule asked them to, and they did, believing that Hercule was the one fighting Buu.

Kid Buu caught the Spirit Bomb as Goku threw it at him and sent it back at him. When Vegeta had Dende use Porunga's third wish to restore Goku's energy, Goku was able to send the Spirit Bomb back at Kid Buu and destroy him once and for all.

Before destroying him, Goku said he would like him to return as a better person, as he was his most worthy opponent yet. King Yemma heard this and, after Kid Buu's death, reincarnated him into a human child named Uub, who unlike his previous self, was not evil.

Trivia

 * Not counting Majin Buu who's the good counterpart, Evil Buu, Super Buu and Kid Buu are the same person but in different forms.