Kaido is toughest without questions because of his DF and skin, so he is a tank, but talented haki top tiers will defeat him. he doesn't even has future sight or even not very good observation. even BM who really sucks at observation haki was better than him. she warned him about Zoro's Enma.
I would assume Shanks , Mihawk and Dragon all of them can defeat Kaido and are harder to fight than Kaido.
I assume Shanks is better in every color of haki than Kaido, imagine he can see further than Katakuri in the future so Kaido will not really be able to lands many attacks on him.
so even if Luffy defeats him he will not be the strongest yet.
Shanks and Dragon as haki masters should be still above him.
and BB will surpass Kaido and even WB because of his mastery of his DFs.
so Luffy will be always the underdog.