Dragon Ball Z is infamous for its ridiculously powerful fighters, but no one ever truly surpasses Goku. He's the character with the most forms, upgrades and powerful attacks, and even when he's not the one to throw the last punch, he's still instrumental in beating the bad guys. This was obviously the case for the series' final saga, the Majin Buu Saga.
However, early on in the fight against Buu, Goku could've killed him with his new Super Saiyan 3 form, but didn't for...reasons. Goku's reluctance to jump at the chance to beat Majin Buu may have stemmed from his desire to see if the other Z Fighters relied on him too much. Though this is just speculation, it does tackle one of Dragon Ball Z's biggest problems, especially concerning characters that aren't Saiyans.
How Goku Let Buu Go
Though he was dead at the time, Goku came back to help out against the threat of Majin Buu by the permission of King Kai. He had undergone intense training in his time away from the land of the living and hoped to use these new skills and abilities against his pink opponent. Upon turning Super Saiyan 3, Goku begins to quickly overwhelm Majin Buu, who oddly still views the beating as a fun game.
Goku himself plays around with Buu, not taking the fight as seriously as he should have early on and testing Buu's strength. He even commends the villain for his ability to copy his attacks, noting that it's a useful skill. This drains the power that Goku was using to stay in the world of the living as a spirit. Before his power runs out, Goku makes a deal with Buu and Babidi to let Piccolo, Goten and Vegeta's son Trunks train in order to fight Buu, before departing for King Kai's Other World.
Why Goku Let Buu Go
This entire affair seems to be a plot contrivance made to extend the saga's story and not just have Goku beat Buu in their first encounter. This could still be true, especially given the fact that Dragon Ball Z was artificially extended beyond Toriyama's original plans anyway. Goku's wanting Goten and Trunks to be the one to beat the bad guy, however, comes from a logical place.
Up to his point, many if not almost all of the series' victories had been won by Goku, who was the last man standing against insurmountable odds. Even he had probably begun to notice this and sought to end the Z Fighters' and the Earth in general's reliance on him for survival. His leaving Buu for the kids to contend with was a push to make them step up, train harder and deal with the threats that he had been taking care of.
Not only would this have been a good idea as a father, but also as a hero, as it left a greater line of defense than the one held by the now older and more domesticated Z Fighters. Of course, it didn't quite pay off the way he intended. Not only did were the kids unable to beat Buu on their own, but there were mass deaths caused by Majin Buu's growing strength and rampage. Of course, they were all brought back by the series' end, but these events certainly make Goku's choice of playing around with Buu at first more than questionable.
