As I alluded to, I'm not a massive boxing historian. I thought of Willard and that was all I could come up with, apart from the obvious ones from the same generation, for fighters that far back in the sport's history.:good Also one of the main reasons I posted the thread:smooch.
Paul Williams comes to mind...He throws a lot of punches and I very seldom see him tire out! Floyd Mayweather is another fighter who is always is tip-top shape.
Early in his pro career, 'Vander's stamina problems were common knowledge. Duva and Benton have commented on this several times. He had a major fast-food addiction, he had near fainting spells after sparring on several occasions. Obviously, 'Vander got it together but he had work very hard to do it.
I don't know if "best conditioned" fits, but Tapia amazed me with his energy in the ring...even 'flipping' after the fight.