He just pictured the most impactful people he encountered, who also happen(ed) to be the strongest of their times. But still, Rayleigh's hype is great. Considered to be not Roger's subordinate but his partner, a name that makes everyone tremble in fear and respect (including Jinbe, who gave a shit about Big Mom), with Garp saying it isn't wise to take on 2 legends at once when referring to Whitebeard and Rayleigh. With Rayleigh even when old and rusty, stalling a marine admiral (the WG's strongest military force).
He's a conqueror, he knows barrier Haki and internal destruction Haki. He most likely can coat his attacks with CoC, given Zoro at this stage could achieve that too while still not being on prime Ray's level.
Tengu said Oden couldn't achieve a black blade, implying that is a rank only the strongest swordsmen can achieve. Now one could ask, why didn't Roger have a black blade. I think because he fights differently, while a black blade is the epitome of cutting. "To cut everything and nothing". Unlike Roger, who throws Haki barriers with his sword, Oden is a pure swordsman in the sense that he is focused on powerful cuts, no barriers whatsoever. That's the exact same fighting style Zoro and Mihawk have. Literally, there is no difference in their fighting style.