It was made pretty clear during Dragon Ball’s Majin Buu Saga that Buu was not to be messed with as even Goku was barely strong enough to take him down, and even then, he had to siphon power from every living thing on Earth to do so. Buu has killed gods, sent others running scared, and threatened the lives of every creature in the universe–but Dragon Ball GT confirms that there is one Z-Fighter Buu wouldn’t be able to beat, and it’s not Goku.
Long ago, before the events of Dragon Ball, Majin Buu was a being of pure evil and chaos that was unleashed upon the universe and threatened its destruction. Even gods couldn’t take Buu down, including the Lord of Lords 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 was successful in freeing Buu during the events of DBZ’s Majin Buu Saga–and when he did so, Buu wasted no time in revealing exactly why he was imprisoned in the first place. Buu not only struck fear into the hearts of literal gods who called upon Goku and Gohan for help, but Buu actually succeeded in blowing up the Earth and then tracking the Z-Fighters who survived the explosion to the world of the Kais. Following a painful fight, Goku was finally able to take out Buu with a Spirit Bomb after New Namek’s Dragon Balls restored Earth. However, in a one-on-one battle, it was clear that Buu would have won–something that wouldn’t be immediately obvious if Buu went up against a brand-new fighter in Dragon Ball GT.
In Dragon Ball GT season 1, episode 19 (written by Atsushi Maekawa, directed by Osamu Kasai, produced by Toei Animation), Goku, Trunks, and Pan are taking a break from hunting Black Star Dragon Balls to visit a planet that has been fully assimilated into the Machine Mutant collective. Machine Mutants are alien Androids created for the sole purpose of conquering planets, and the strongest among them is a villain named General Rilldo. In this episode, the Z-Fighters are brought face-to-face with Rilldo, and they weren’t immediately confident they were going to get out of this one alive since, according to Goku, General Rilldo is even stronger than Majin Buu. However, right at the start of the fight, Pan is the first one to throw a punch against Rilldo–and the results are shocking to say the least. Pan knocks Rilldo to the ground in just a few debilitating hits, and he stays down for some time afterwards.
With More Training, Pan Could Destroy Majin Buu
Pan took down a villain who was more powerful than Majin Buu without reaching even a fraction of her full power. At this point in her life, Pan couldn’t even go Super Saiyan despite the fact that her power-potential was enormous which proves that she still had a long way to go before becoming the strongest she could be. So, the fact that she knocked Rilldo to the ground with relative ease during perhaps the weakest point in her life is incredibly significant. If Pan were one of the Z-Fighters challenging Buu during DBZ’s Majin Buu Saga, this scene in GT indicates that she could have been the one to defeat him, given enough time to train.
During the Majin Buu Saga, the Z-Fighters were utilizing the Hyperbolic Time Chamber which allowed them to fit years-worth of training into just a few days. If Pan can take down someone who’s stronger than Buu before reaching her peak, it stands to reason that she’d beat Buu himself after years of hardened training in a room designed to make one stronger. So, with all the evidence present in GT, it seems clear that Pan–with more training–is the one Dragon Ball Z-Fighter (aside from Goku) that Majin Buu couldn’t beat.