I's a tough thing to be objective about IMO.
How do you compare fighters from way back in the 20s, 30s and 40s like Harry Greb, Gene Tunney, Willie Pep, Joe Louis, Sugar Ray Robinson etc, with modern day boxers considering the advances in conditioning, and how the sport has changed and evolved?
Generally how P4P and GOAT lists are worked out is not really made clear in terms of what the criteria is.
MY criteria would be as follows:-
1. Resume - did they fight the best of their time and how did they fare? Undefeated? Avenged defeats?
2. Prime Factor - how 'unbeatable' were they at their peak?
3. Adaptability - multiple weights? Plan A,B - Z?
4. Legacy - did they leave the sport at the right time, or did they tarnish their legacy by fighting on too long?
5. Intangibles - this is just that 'je ne sai quois' that can separates the best from the rest.
Now, based on that criteria, in the modern day with the farce that is boxing politics, and snakes like Al Haymon and Bob Arum, how do we get the best to fight the best? Resume in the modern day is undermined a lot #, and it's not necessarily the boxer in questions fault.
My objective GOAT list based on the above criteria is
1. Sugar Ray Robinson
2. Willie Pep
3. Muhammud Ali
4. Joe Louis
5. Harry Greb
My favourite boxers on the other hand are
1. Nigel Benn
2. Joe Calzaghe
3. Manny Pacquiao
4. Muhammud Ali
5. Roy Jones Junior
Even though I loved watching Zags fight, I agree with folks saying that Calzaghe can't really get anywhere near top 10 - I'd even say top 20 on the GOAT list is a stretch. His resume simply isn't good enough - whether that is Frank Warren's fault or whatever I don't know, but it's true.
Compare Carl Froch - whilst I'd take Calzaghe to win a wide UD everyday of the week and twice on Sunday, Froch arguably has a better Resume.
And then Floyd - there is so much corruption and politics in boxing and Floyd has so much power and influence, IMO his Resume is questionable as well. Because he is the 'A' side in every negotiation, he always stacks the odds in his favour; Ring Size, Gloves, etc etc. Even with Marquez, coming in over weight.
I admire Floyd's ability, but I think he falls short on GOAT lists for a number of reasons.