Majin Buu, in canon, is depicted as being a primal embodiment of Chaos, Evil, and Destruction up until the moment he absorbed Daikaioshin-sama.

Which honestly makes sense, when you consider that his powers clearly don't follow the same rules as the magic and ki techniques of other people: As versatile as ki and magic is, Buu just kind of bends reality and if you've been keeping up with the Super Manga, per the Planet Eater Moro Saga(which, incidentally, confirms that Magic and Ki run on the same juice,) Buu's energy isn't ki or life energy, or at least, isn't subject to the same limitations as ki and life force(Moro, who uses other people and planets ki and life force to power his spells and drains it to increase his power can't feed on Buu.)

So, if he needs an element, Chaos.

Or Maybe Buu is just the original Majin, they're all Chaos, but the others are chaos reflected through different concepts which trade some of their chaotic powers for power over that concept.