in pure 1v1 combat with JUST swords and haki, Mihawk HAS to be the strongest there is for Zoro to be the strongest swordsman of all time.
Therefore Xebec, Roger, Shanks are all bellow Mihawk and black blade proves it.

Mihawk will be Zoro's 2nd strongest enemy before his final opponent where he will enter as the strongest swordsman but there will be another swordsman with black blade, only he won't be using just haki and swordsmanship.