In the anime-filler flashback, Dai Kai held his own against Buff Buu for a while. Also, supposing the absorptions are just additions/substratcions of power unless stated otherwise, considering the power Buu lost when he absorbed Dai Kai was more than the power he gained by absorbing South Kai (since Fat Buu was weaker than Kid Buu), I think overall it makes more sense to treat Dai Kai as the strongest. Besides, he was at the top of the hierarchy.