During the Buu Saga of Dragon Ball Z, the titular villain Majin Buu absorbed not one, but two Supreme Kais during his reign of terror. While he is now on the side of the Z Fighters, back then he was prone to absorbing others in order to gain their powers and strength.
However, now that Buu has changed for the better in Dragon Ball Super, does he still hold the power of South Supreme Kai and/or Grand Supreme Kai?
Five million years ago, the sorcerer Bibidi summoned the pink villain in his most natural Kid Buu form. While Kid Buu was powerful in his own right and was able to kill North Supreme Kai and West Supreme Kai, he was badly injured during the fight against South Supreme Kai and was forced to absorb his opponent to gain his power. Because Buu's form changes when he absorbs someone, he transformed into a massive, Hulk-like being. Soon after, he also absorbed Grand Supreme Kai when the latter protected Shin from an attack, giving him the round, goofy ball of pink that was first introduced to the Dragon Ball Z audience.
This version of Buu, dubbed Innocent Buu, is said to be more powerful than Kid Buu, but because of Grand Supreme Kai's good nature, that strength had been partially stunted. The danger of Kid Buu comes from his recklessness and unpredictability, rather than pure physical strength. So while Innocent Buu has the power of two Kais on his side, Kid Buu is more of a wild card and thus poses a greater threat.
After Super Buu absorbed Piccolo, Gotenks, Gohan, and Innocent Buu, Goku and Vegeta allowed themselves to be absorbed as well in order to fight from the inside and free everyone. This allowed Kid Buu to reemerge because the influence of Grand Supreme Kai was no longer there, as it remained a part of Innocent Buu instead. However, South Supreme Kai was still fused with Kid Buu, so when the latter was finally destroyed, so was the former.
Grand Supreme Kai's spirit lives on in the form of Innocent Buu. This is confirmed in Dragon Ball Super when the Galactic Patrol uses hypnosis and aromatherapy to trigger the latent memories of the Kai's fight against Moro 10 million years ago in order to have him fight once again. So while the primary personality is that of Innocent Buu, the memories and powers of Grand Supreme Kai remain within him.
Thanks to Mr. Satan and the influence of Grand Supreme Kai, Buu is no longer a major threat to the universe. It also helps that most of the Godly Ki was infused into Uub when he was reincarnated from Kid Buu. The mechanics of how the split occurred aren't completely clear on why the Godly Ki went to Kid Buu instead of the part that Grand Supreme Kai remained with. So while Innocent Buu still has some Godly Ki, he doesn't know how to tap into it if Grand Supreme Kai isn't in control of his body. It also remains unknown whether South Supreme Kai's power also transferred to Uub, or if it died with Kid Buu.