My Two Cents:
Muhammad often claimed (hype or not) to be the greatest of all time, which implied P4P. There was a time he seemed (to me at least) to be the greatest of all time in his weight class. In hindsight, I've gained enough perspective to give him this: Ali was the greatest of his time (or era) among heavyweights.
In summation and in my book, Sugar Ray Robinson holds the title of Greatest of All Time, P4P. Joe Louis was the greatest heavyweight ever, with Muhammad Ali a close 2nd.