Majin Buu is the final villain of Dragon Ball Z and, in his good version, one of the most constant presences in everything that follows: in GT he merges with his own reincarnation, and in Super he is part of Earth's team, even if he has a habit of falling asleep at the worst possible moment. The full portrait, forms and powers included, is in Majin Buu's profile; here we follow his path arc by arc.
Dragon Ball Z
Majin Buu Saga (episodes 232-291)
Buu has slept, sealed on Earth, for millions of years, ever since the Supreme Kai killed the wizard Bibidi. Bibidi's son Babidi gathers the energy he needs during the 25th World Martial Arts Tournament, and the clash between Goku and Majin Vegeta does the rest. In the Majin Buu Saga the cocoon opens, and in episode 232 the pink smoke takes shape: a chubby, childlike creature that looks harmless. Buu flattens Gohan and the Supreme Kai, turns Dabura into a cookie and eats him, then survives Vegeta's self-sacrifice by regenerating from the fragments. Goku holds him off as a Super Saiyan 3 and points out how badly Babidi treats him: as soon as Goku leaves, Buu kills his master.
Free, he keeps destroying cities for fun, until Mr. Satan tracks him down with poisoned gifts and booby traps that Buu doesn't even notice. A friendship grows between them; Buu heals an injured puppy, Bee, and promises to stop killing. When two gunmen shoot the dog and then Mr. Satan, in episode 254 his rage splits him in two: out comes a thin, evil Buu who turns the good one into chocolate, eats him and becomes Super Buu. This version wipes out almost every human left, absorbs Gotenks and Piccolo, and then Gohan too. Goku and Vegeta, fused as Vegito, let themselves be absorbed on purpose and, from the inside, pull the good Buu free: Super Buu reverts to his original form, Kid Buu, who immediately destroys the Earth.
The final battle moves to the Supreme Kai's world. The freed good Buu lends Goku and Vegeta a hand, and Mr. Satan convinces the revived people of Earth to give their energy: in episode 286 the Spirit Bomb erases Kid Buu, while Goku wishes for him to be reborn as a good person. The good Buu settles down with Mr. Satan and Bee, and ten years later, at the 28th World Martial Arts Tournament, Goku recognises the evil Buu's reincarnation in a boy named Uub and leaves to train him.
Dragon Ball GT
Baby Saga (episodes 28-38)
When the parasite Baby takes over the Earth, Buu spits out the egg planted inside him and protects Mr. Satan, the only other person to escape the infection; to keep them safe he even swallows Mr. Satan and Pan. In the Baby Saga the three reach Baby's planet, where Uub is losing. In episode 32 Buu takes Baby's huge energy ball head-on, explodes, and his fragments rain down on Uub: the two merge for good into Majuub. Baby turns him into chocolate and eats him, but Majuub torments him from the inside and buys Goku the time he needs.
Super 17 Saga (episode 41)
At the 30th World Martial Arts Tournament Majuub reaches the final against Mr. Satan and is about to win, but Buu's voice, from within, asks him to let it go: the world needs its champion. That's the story of episode 41, which opens the Super 17 Saga.
Dragon Ball Super
Battle of Gods Saga (episodes 6-7)
At Bulma's birthday party Buu refuses to give Beerus the last pudding. In episode 6 the God of Destruction loses his patience, and in the Battle of Gods Saga Buu is swept aside along with the other fighters.
Universe 6 Saga (episodes 30-32)
Buu agrees to fight against Universe 6 once Goku promises him a wish on the Super Dragon Balls, which he wants to give to Mr. Satan. In the Universe 6 Saga, however, he falls asleep during the written exam and in episode 32 is disqualified before ever stepping into the ring.
Universe Survival Saga (episodes 78-92)
In the exhibition match in front of Zeno, Buu faces Basil of Universe 9: he treats it as a game until Mr. Satan gets hurt, and then, in episode 79, he beats his opponent and heals his friend. He trains so hard for the Tournament of Power that he slims down, and in episode 85 he even beats Goku in a sparring match. That very effort, though, sends him into a sleep of about two months: in the Universe Survival Saga his place on the team goes to Frieza.
Movies
In Battle of Gods (2013) Buu has a small but crucial role: over a pudding, he is the one who unleashes Beerus' fury at Bulma's party, the same scene Super revisits in its first arc.
Did you know? Dragon Ball Daima revealed that Buu was created by the witch Marba at Bibidi's request, and in episode 8 Marba uses Buu's remains, collected after Vegeta's explosion, to bring a new Majin to life.
Our take
Followed arc by arc, Majin Buu takes a journey no other Dragon Ball villain completes so fully: the ultimate threat splits in two, and the good half becomes part of the family. The turning point isn't a fight but a puppy and a fake champion who stops being afraid.
The later series choose different paths. GT gives him his most moving ending, sacrificing himself for Uub; Super mostly uses him for comedy, but the win over Basil is a reminder that when someone hurts Mr. Satan, Buu gets serious.
For the fighter who takes his place in the Tournament of Power see Frieza saga by saga, and for his best friend Mr. Satan's profile.