The theory I present to you is based on this scheme (sorry about "Grand Supreme Kai", who is the same as Dai Kaioshin):
And what it means in terms of power, using numbers on a smaller scale, is that:
Evil Buu (10) = Buff Buu (10) > Super Buu (6) = Kid Buu (6) > South Kaioshin (4) > Fat Buu (0) > Mr. Buu (-4) > Dai Kaioshin (-10)
Kid Buu + South Kaioshin = Buff Buu
6 + 4 = 10
Buff Buu + Dai Kaioshin = Fat Buu
10 + (-10) = 0
Evil Buu + Mr. Buu = Super Buu
10 + (-4) = 6
Super Buu - Fat Buu = Kid Buu
6 - 0 = 6
Extras:
Kid Buu + Dai Kaioshin = Mr. Buu
6 + (-10) = -4
Fat Buu - Mr. Buu = South Kaioshin
0 - (6 - 10) = 4
Fat Buu - Dai Kaioshin = Evil Buu
0 - (-10) = 10
Reading this scheme and these formulas, we can deduce that:
Super Buu is Evil Buu with Mr. Buu absorbed, meaning that he is evil and has suppressed power. Evil Buu being stronger than Super Buu only means that Super Buu has the potential to get stronger by tapping onto his anger, like Mr./Fat Buu did.
Mr. Buu is Fat Buu without his evil strength, who first appeared when Fat Buu released Evil Buu. If Evil Buu's strength should be based on Kid Buu plus South Kaioshin's essence, then Mr. Buu's is based on Kid Buu plus Dai Kaioshin's.
Buff Buu is Kid Buu with unsuppressed power given by South Kaioshin. Once Fat Buu is disconnected from Super Buu, Kid Buu's persona takes over as his original self, which means he has not tapped onto South Kaioshin's power, nor is suppressed.
Evil Buu is based on Fat Buu without Dai Kaioshin's suppression, so his power is the same as Kid Buu's with South Kaioshin absorbed. Once Evil Buu absorbs Mr. Buu, he is suppressed and turned into Super Buu.