The thing is, it's completely subjective. Just like comparing any great boxer from today to any in the past.
One thing about Ali, is by far and away, he faced tougher opponents than Floyd. I am NOT saying Floyd hand picks guys or faces shitty fighters. It's just the the golden era was full of dangerous, high level HW fighters. Ali fought them ALL. When you are fightng the absolute best possible guy every time you're out there, the chances of losing are a lot higher.
Let's not forget, Ali missed 3 and a half years of his prime when he skipped the draft. THREE AND A HALF YEARS! That's like Dominick Cruz level of absence if not more.
IMO, wins over
Sonny Liston x2
Prime Foreman when he was an unstoppable machine that no one even came close to beating
Frazier twice
Ernie Shavers
Ken Norton
Leon Spinks
Floyd Patterson twice
Jerry Quarry
Henry Cooper
Cleveland Williams
and more
Floyd's best wins are over
Vargas
Corrales
Castillo (and he LOST that first fight, too)
Oscar
Hatton
Mosley
Cotto
Canelo
Those guys are all awesome, great fighters. Champions. But how many of those guys are going down in the all time great list? Not many.
But Almost everyone I listed in Ali's names is a hall of famer, all time great. Liston, Foreman, Frazier, Norton, Patterson... etc. Floyd has never really beaten a guy that was as reputable as them.He has beaten great fighters. I'm a HUGE Floyd fan. I don't hate him because of his personalty. I love him because of his skills in the ring, and his defense that is like no other.
But just because Floyd never lost does not make him the greatest automatically. Rocky Marciano is not the greatest HW ever, but he's up there. He's not the clear cut, #1 guy, though, just because he never lost.
Ali's quality of opponents trumps Mayweathers. IMO, at least. Also, Ali finished A LOT of guys. The fact he finished Foreman when Foreman was KILLING EVERYONE, including knocking Joe Frazier down 6 times n 2 rounds... No one ever expected that. Floyd has never cme back from that kind of adversity to pull of a miraculous KO against a guy on equal level to him.