I feel like Prime Rayleigh could be an absolute powerhouse, maybe even above Yonko level. Think about how well Rayleigh was able to fight Kizaru at age 76. His performance was far better than any 1st Commander that we've seen against an Admiral, and that's after his power has taken a massive hit. Whereas we saw Luffy have some trouble with Kizaru despite being stronger, implying the Yonko aren't that much further ahead of the Admirals. Prime Rayleigh could be stronger imo.