Sugar Ray Robinson long ago started being regarded as the all-time pound-for-pound best professional fighter ever. I believe he's stood the test of time. Almost 200 professional fights, multiple titles and defenses in multiple weight classes (at a time when weight classes meant something)...sheer talent.
All of the others had their high points and earned their time in the spotlight. Ali, in particular, helped to resurrect boxing at a time when it had fallen into the trashcan. And, before they stripped him of his title, he was a thing of beauty and a joy forever... But Robinson was the best.