Leaning towards BM winning with very high diff for now.
Aokiji was apparently taken out for an extended period of time by Garps initial attack against him and Oda injured Garp via a cheap shot before he came back into the fight. This does make me think that Kuzan is ~ old Garp at absolute best, potentially weaker.
We see in the flashback that he was able to one shot almost 2/3 of Blackbeards crew. Garp got weaker for sure (his words) but it just means Old Garp is admiral level.
Big Mom making Homies on the fly to give her numbers while also being a monster herself physically with regen I feel like would give her a win 99% of the time without pis