I don't disagree with your basic premise but I would disagree with some of your candidates. Joe Louis is one of the greatest heavyweights ever but on a pound-for-pound basis, he's not near the top. IMO, Roy Jones is over-rated and over-hyped. Really good but I think there are better. It's way too early to put GGG in that group. Duran, I agree. In following boxing for over 50 years, Roberto Duran in the mid-70s was the best I ever saw, PFP. If Duran had been 215 pounds with that skill set, he'd easily beat Ali.
Sugar Ray Robinson has to be on an all-time list of the greatest PFP fighters. He'd be at the top of mine.
Ali was a great fighter and champion but on a PFP basis, there were better fighters.