King K.Rool would beat bowser, hands down. Why? He can charge him, stomp him, throw a fast-flying crown at him, and crash multiple cannonballs into him. If all else failed, then he could turn into his alter-ego, Cap'n K.Rool, where he could then launch mines into him, slow him down with gas, or make him move in reverse to trip em with reverse gas, shoot him with his gun, or stomp him again.
In short, King K.Rool ftw.