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It's totally subjective. I could say Abner Mares is the P4P best fighter in the world, it's not right or wrong, it's an opinion.
Floyd is the consensus #1 b/c he's fought high level competition for years and never lost.
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No scientific method to judge.
Jumping many divisions and not loosing is always a good way to top the list, it is how Floyd did it.
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19 years undefeated world champion