They are no longer rivals but in the narrative, the Oda leaves it ambiguous which of them is stronger.
On the one hand, Mihawk has the WSS title and Shanks is served as a swordsman throughout the Manga, but on the other hand, we only received a statement that Mihawk is superior to him in swordsmanship and we donβt know how their duel ended (presumably a draw)
Something like this.