Liston didn't fight in '57 and when he came from years of inactivity in '58 he'd only had 15 fights. So i'd say he was still developing thorough '58 ad '59. He looked better against Williams in '60 than the previous year.
With Muhammad Ali's career stopped at that point I will have to go with Rocky Marciano's wins. Louis - Walcott - Walcott - Charles - Charles and others to top Liston - Liston - Patterson and others. (If I add in Muhammad Ali's second career wins then I am going with Ali. He has Liston - Liston - Patterson - Patterson - Frazier - Frazier - Foreman - Spinks and others.)
"High quality victories" ?!? WTF are you talking about ?!??????????????? Liston was way older than stated and did not take Ali seriously, so he came unprepared for the first fight. Got injured during it. Took a dive in the rematch. Patterson had a broken back. Those victories were anything but high quality.
In his entire career or just the 60s? Neither Liston, Patterson, nor Moore were in their prime. In the 70s, he beat a prime Foreman, and Norton 2X.
I agree Liston was not at 100% and past his prime. That being said, calling this victory irrelevant is beyond absurd for reasons I don't have to state.
I think with the wins Floyd had going in to the fight and seeing how he'd be competitive at the top level for another 7 years that you'd have to say he was about prime. The back injury likely hampered him for the second half of the fight, but going into the fight I probably wouldn't call him a past prime fighter. As for Liston... I kind of hesitate to venture into that territory since it has lead to several endless discussion already. Suffice it to say that I'm always quite hesitant saying that someone who has looked awesome in his previous fights is past his prime. There's better reason to claim that he took Ali too lightly and at least was bothered by his shoulder. In the rematch, what little we got to see, I do think he looked like a past prime fighter. At the very least lacking in confidence.
We've gone into this countless times, and if we engage in it further it'll probably go the length of an ESB Marciano thread so I agree it's best left alone.
Liston and Patterson at that point are still better fighters than the corpses of Joe Louis, Joe Walcott, Moore, and Charles that Marciano beat. Marciano beating those fighters is like Ugas beating Manny Pacquiao. That's how washed up they were.