Well, with Zero feats from Kaido, imma say Akainu. Dont bring that "Strongest (Something) in the world" crap because that kind of hype allways has loopholes.
Tie.
Unfortunately we only have Akainu's performance at MF (which was shaky at best vs WB) and Kaido's loophole filled title of "strongest whogivesadamn in the world."
People should keep in mind that it makes Kaido too OP becuase if Kaido is the strongest in the sea that means that Kaido in a pool of sea water>Shanks which i would not even question Shanks rofl stomping him.