The databook referred to Shanks as top class swordsman. And again, he was Mihawk's rival. Do you think Mihawk considers someone his peer and rival in swordsmanship which he honors, who just randomly picked up a sword which he exchanges for a gun on occassion
And again, what abilities could Shanks have apart from swordsmanship with one arm and without a DF? So isn't being a top class swordsman, user of a supreme grade sword and being Mihawk's rival enough? What else do you need?