I'd dispute Mayweather and Hopkins, I think they preserve energy very well to fight strong by the 12th. Their activity is low so I wouldn't consider them the most conditioned, won't know for sure until someone makes them work more but they're greatly conditioned either way.
In the Hopkins-Trinidad fight we got to see how incredibly conditioned Hopkins was. Remember that Trinidad had ensless stamina and is how he won many of his fights towards the latter part of fights.... ...Hopkins however was fresher and had outworked Trinidad throughout the entire fight.
Jack Johnson Floyd Mayweather Carlos Monzon Ray Robinson Stamina is such a huge, even underrated, part of boxing. No coincidence that this list will comprise of absolute legends.
Sal Sanchez by far had the best lungs who you never saw gasp or pause for air, Marciano, Hagler, and Calzaghe.
Guys with low workrate need to be automatically taken out of the question. As are guys who don't sit down on their punches. The answer to this question has to be a hard volume puncher. Guys like Hagler or Pacquiao.
Im sorry, but no. The video's of him "weightlifting" are cringeworthy. His 200kg "deadlift" I was waiting for his back to go ping. Terrible form through out a majority of his training. There are multiple videos of his training that do him more bad than good. He cannot be compared to someone like Floyd, Bradley, Pacquiao, Zaggers and Hopkins (to name a few). Regardless of his size.