Mihawk back in prime I believe was the WSS, pure speculation but it is plausible. He was a swordsman, one of the strongest in his time, he could have been the WSS. So prime Rayleigh might win, as even when he is weak and old, he is taking on someone like Kizaru. So b/w prime Rayleigh and Mihawk, Rayleigh.
As for now, he had been retired for a long time as he himself said while fighting Kizaru, and obviously he's now old. So currently Mihawk. Although fighting Kizaru on par is a good feat(even the current Rayleigh is a little stronger than Kizaru IMO), but Mihawk could have done that too with less difficulty. why? WB=>Mihawk=>Shanks>An Admiral(WB defeated the strongest one).