Some considered Sonny the best HW ever before he met Clay. Until Joe Frazier came along there was no one who could challenge him except Liston himself. That's the Mike Tyson case. At some point you stop improving and start regressing but if Frazier had beaten Sonny around 1968-69 and then been the dominant champion himself until 1973 it would have been seen as a natural generational change. Sonny would probably be in the top 5, considered by many to be the Goat who just got old. Of course there is plenty of evidence that Liston like Tyson lost focus on boxing so it is quite possible that he would have been beaten by someone like Ernie Teller who was nothing special. But if he caught Liston at the right moment in his career and then probably lost to Frazier, today we would be arguing whether Liston was great or simply didn't have a really good, tall opponent with a jab and grab style to fight Terrell. The historical approach can be very distorted. A lot depends on the circumstances that are forgotten after years. Personally, I think Liston's career was one big misfortune, just like his life, he simply had no luck or no one knew how to help him.
Anyone who thinks Liston gets Frazier in 66-67 is hysterical to me LOL - Liston after 64 was a blimp, big but mostly hot air he was collecting the easiest purses he could and strangely didn’t fight a single credible opponent for a title shot in the last 5 years of his career.