Luffy's win over Kaido is one of his most significant achievements since One Piece started. By taking out the Strongest Creature, he took his place as an Emperor of the Sea, earned a 3 billion Berry bounty, and established himself as one of the strongest characters in the series. Unfortunately, many consider this win invalid for one reason or another. Some say Kaido lifting the island stopped him from fighting at full potential. Others say fighting multiple opponents softened the King of Beasts up for the Straw Hat captain. The general assumption is that if these two were in a one-on-one fight with no outside help or interference, Kaido would have easily beaten Luffy.
Source：https://www.cbr.com/one-piece-luffy-vs-kaido-fair-fight/
