Putting Mihawk above an admiral is to much, those 3 dudes fought evenly with WB,Akainu actually having had the upper hand in most of the fight, while Mihawk stated during the war that he wanted to see the difference in strenght between him and WB, meaning he believed not to be on his level..
And I dont think Mihawk is stronger than Shanks, it was said they used to duel before shanks got injured,so we dont know if the title of the strongest was only claimed by Mihawk after Shanks lost his arm, or even if Shanks had any intention on being granted such a title while he still had both his arms.
He is,no doubt,incredibly strong, and will probably be one of the last, if not the last, big opponent of Zoro,but I dont even think Zoro will end the manga being capable of defeating an admiral 1 vs 1, they are just that powerful,I even doubt Luffy himself will beat any admiral 1vs1 except,perhaps, Akainu, because of revenge,and that will be pretty close to the end of the manga,considering how difficult it is to reach Akainy nowadays.
So, Mihawk is probably the strongest of the shichibukai,depending on Flamingo (from whom we've had so little information) and belongs on the tier of strongest characters in the manga, aside from the final antagonist/Admirals.
I do not doubt he could evenly fight with any Yonkou ( captain ), but I personaly dont believe the captains of the yonkou crews are the strongest characters in the manga, they are very strong,but are only the top 4 because of the crews they've gathered,well, aside from Blackbeard who is being hyped to be the strongest character Luffy will have to defeat..