Black blade.
I think Mihawk>Prime Ray because Mihawk represents EoS Zoro's goal.
Zoro would be what Ray was just by being the rhm of Luffy's ship. However, there's a difference between Zoro and Rayleigh. Zoro has had a life long ambition of becoming the WSS.
Ever since he was a child he has had that drive to sail the seas and become the strongest
He has always had a clear cut goal and drive to achieve his goal and become the strongest one day
He vowed to become so strong and renowned as a swordsman that his name would be famous in heaven itself
Compare that to what we know of Rayleigh. When he and Roger meet Rayleigh is living in a stolen ship, anchored at some unknown place, because his house burned down.
He isn't doing anything like Zoro, who was actively seeking Mihawk and beating people on his way to his goal. He was just chilling somewhere, without a care in the world, and without any hint of any ambition or goal in his life.
To further illustrate this, when Roger tells him this is a meeting of fate and they should turn the world upside down, Ray's first reaction is to tell him to get lost. Basically, Ray just wanted to chill there for as long as he could, without a care in the world.
Which is also further emphasized by the fact that when we meet Ray he is basically doing just what he had done before joining up with Roger. Keeping a low profile on some island, drinking away, gambling, banging chicks, stealing, enjoying life and just waiting for his death. However, upon meeting Luffy, Zoro, and the rest, he has found something to look for, something that would make him wanna live life again fully and see with his own very eyes what will happen.
So basically, if Zoro was just on the PK's ship as the rhm he'd surpass Rayleigh as it is, but he goes a step beyond that and surpasses Mihawk. So Mihawk>prime Ray.