Well, each of them has some of the strongest Haki known, so it's a tough call without seeing them go toe to toe. I'd side with Shanks, though. For one, he's quite a bit younger. The major reason, however, is that he's a Yonko, and (to our knowledge) the only one without a Devil Fruit power. One doesn't acquire such an infamous title without some incredible strength, much which is acquired through his substantial Haki. Rayleigh certainly has quite the title as well, but not one that's nearly so substantial, and that makes it at least less likely that he's of higher strength.

I would more likely pick Rayleigh, as he has had a lot more years of experience than Shanks and, even if he's old, that doesn't really affect the usefulness of his Haki, just the damage his body can take and probably reduced stamina. However, we've seen both of their Haki and it didn't seem that either was more powerful than the other, or rather, their skill seemed to be fairly even.

I chose Shanks because he is a yonko without being a devil-fruit eater.