I would agree Kaido's fleet may be the weakest. I wouldn't say Black Beard is on the top. Reason so BB could easily just over take and defeat Shanks/Crew. Big Ma'am doesn't seem the sort to interfere if a battle occurred. The only issue here is Law saying Kaido being the strongest. BB is too ruthless to allow Shanks any room for comfort. He would rather deal with people more to his liking and trade.
2. Black Beard
3. Big Ma'am
Hoping Oda follows suit and it would make sense story wise
Kaido first cause duh..fits in with current Arc
Big Ma'am second because she will corner Luffy/Law crew in the NW and the second arc of the NW will take place after Dofla, Kaido. Then Luffy heads out to take on BB for his brothers sake. Taking on new Territories Luffy will build even more recognition. Maybe at this point he has already turned down an offer to be apart of the WG-12. Then lastly fighting Shanks, his friend and last hill to climb...
Knowing Oda though this would probably make too much sense and throw us for a loop