Manga Stream explained it better.
Kaido is strongest creature, not strongest pirate/man alive. That was also said like 10 years ago.
"Creature" imply that it's animal (from which humans are usually excluded).
So most likely Kaido's title strongest creature refer to him being the strongest Zoan user around and not pirate/man.
You are welcome.
Now on topic:
Shanks > Kaido > Big Mom > BB
Reason for Shanks being no1:
- capable of sparring with "Strongest man at the world" (at least at that time) and during that 'fight' sky splits.
- it's possible that he beat Kaido during the MF or that Kaido was affraid to go against Shanks at that time (tho, it's not confirmed)
- Akainu and Marines haven't go against him and his crew when he arrived at MF.
Other three are purely speculation.
I think we have most informations about Kaido and his title strongest creature speak a lot of his strength.
Between Big Mom and BB, I choose BM, mainly for the reason that she is on the position more than BB and for the reason that just 2 years ago BB run from Akainu.