for my theories,I will split dbz in 3 parallel universes:
dbz anime-kid buu is the strongest buu.
dbz manga-the original kid buu,he is weaker than super buu.most of my theories is about him.
dbz gods-if you consider BoG cannon,then kid buu is a god tier.however,he can't use his god level powers at will.
theory 1: you can't measure kid buu's power by ki
because he looks weak? no,they have already learned that lesson,fat buu doesn't have a "strong look".
so,why? because his ki was weak,very weak.maybe weaker than freeza.vegeta thought kid buu can't survive a planetary explosion.
in manga/anime- originally,majin buu was created by bibidi.so,he is an artificial living being,he has ki,but most of his power is "magic".
in dbz gods- a recent interview of akira said buu was not created,he was summoned by bibidi.he is ancient criature that has existed since time immemorial.he might be a demigod and gods have divine ki that can't be sensed.
goku and vegeta understimated kid buu. the kaioshins,however,overestimated him.why was kibitoshin so worried? because he can't know how powerful kid buu was.darkness gives us fear because we can't see what is there.