Luffy is the next pirate king then zoro must be the next dark king, so the level of zoro should be close or at the same level as reyleigh. And reyleigh at his age is still on par with an admiral. So if zoro reaches that level then his greatest rival should be that strong as well.
And I think mihawk can use haki or his sword is made of seastone. It would be lame if you are called greatest swordsman if you cannot cut logias like croc