Note: this will contain spoilers
Luffy as strong as he is even being able to be referred as the fifth emperor could not be able to put a single scratch on kaido. And the funny thing is that luffy was in rage mode and kaido was drunk, yet only one hit from kaido was able to knock luffy unconscious.
Big mama came to the wano kingdom after kaido has warned her, if kaido was giving big mama a warning it is safe to assume kaido is the strongest emperor of the sea.
And against black beard since his devil fruit can stop the devil fruit abilities of various users, it will be useless against kaido, so u see know how strong kaido really is