Dragon Ball Confirms A Z-Fighter Buu Can't Beat (And It's Not Goku)
Although Goku defeated Buu, DBZ made it clear that he couldn't do it alone, but a Z-Fighter hinted that they could single-handedly defeat Majin Buu.
It's clear in Dragon Ball's Majin Buu Saga that Buu can't be pissed off because even Goku is barely strong enough to take him down, and even then he has to draw power from every living thing on Earth to do so. Buu kills the gods, scares everyone else, and threatens the life of every living thing in the universe—but Dragon Ball GT confirms that there's one Z-Fighter Buu can't beat, and it's not Goku.
Long ago, before the events of Dragon Ball, Majin Buu was a being of pure evil and chaos, unleashed into the universe and threatening its destruction. Not even the gods could bring down Buu, including the Lord Lord who was absorbed by Buu and effectively killed in the process. While the ancient warriors of the universe did manage to imprison Buu in a cocoon of seemingly endless sleep, Babidi managed to free Buu during DBZ's Majin Buu Saga event - and when he did, Buu immediately explained why he was in the first place. Time is imprisoned. Not only does Buu frighten those who summon the gods With the help of Goku and Gohan, Buu actually manages to blow up Earth, and then tracks down the Z-Fighters who survived the explosion to Case's world. After a harrowing battle, Goku was finally able to take out Buu with the Soul Bomb after the Neonamek Dragon Balls restored Earth. However, in one-on-one fights, it's clear that Buu will win -- something that wouldn't be immediately obvious if Buu were to go up against brand new fighters in Dragon Ball GT.
In Dragon Ball GT Season 1 Episode 19 (written by Atsushi Maekawa, directed by Osamu Kasai, and produced by Toei Animation), Goku, Trunks, and Pan temporarily visit a planet after hunting the Black Star Dragon Balls to fully integrate into Machine Mutant collective. Machine mutants are alien robots whose sole purpose is to conquer planets, the most powerful of which is a villain named General Rildo. In this episode, the Z-Fighters come face to face with Rilldo, and they don't immediately believe that they'll leave the episode alive because, according to Goku, General Rilldo is stronger than Majin Buu. However, when the game started During the fight, Pan was the first to punch Rildo more - and the results were shocking, to say the least. Pan knocked Rildo down with just a few deadly shots, and he lay on the ground for some time afterwards.
With More Training, Pan Could Destroy Majin Buu
Pan knocks down a villain stronger than Majin Buu without reaching a fraction of her full power. At this point in her life, Pan's inability to even become a Super Saiyan despite her immense power potential proves that she still has a long way to go before becoming the strongest. So the fact that she knocked Rilldo to the ground with relative ease in the weakest moment of her life is very important. If Pan was one of the Z-Fighters challenging Buu in DBZ's Majin Buu Saga, this scene in GT shows that she could have been the one to beat him given enough time to train.
During the Majin Buu Saga, the Z-Fighters use a hyperbolic time chamber, which allows them to complete years of training in just a few days. If Pan can knock out someone stronger than Buu before reaching his peak then It stands to reason that she would beat Buu after years of grueling training in a room designed to make a person stronger. So, based on all the evidence that exists in GT, it's clear that the more trained Pan is the Dragon Ball Z fighter that Majin Buu can't beat (except for Goku).