Hagler, Holmes, Hopkins and Hearns. As for guys without an H in their name, the boxers from Eastern Europe are always in terrific shape.
The non Steroidal ones that do not have to use the juice in a vile, they call it Modern Nutrition, I call it Bologna. Not everyone is going to be as big as the giant in Jack In The Bean Stock. Normal trained fighters who were always in good condition for their fights were Alexis Arguello, Salvador Sanchez, Carlos Monzon, Marvelous Marvin Hagler, Archie Moore, a pre banished Muhammad Ali, Rocky Marciano, and Joe Louis.
Marciano, Hagler and BHOP. I know most people wouldn't have Hopkins but the guy lived like a Spartan warrior. Never put anything bad in his body and was never more than five pounds over his fighting weight.
As far as fighters not listed yet Mancini Tiger Armstrong Pedroza Zale Frazier Tunney Schmeling Ross Olivera Briscoe
Suprisingly, Mike Tyson used to be one of the best conditioned fighters in the world. He lived, breathed, and concentrated on boxing and training. During his peak with Rooney, he could fire away with volume power shots and constant movement for 12 round.
Respectfully disagree...was a little known secret even in his prime his power often disappeared after the sixth (at least his trainers said they knew it). He also didn’t throw many shots per round (think only a little over 30). For a pressure fighter that’s not a lot. Helped that he was accurate and fast.
This kid from Puerto Rico could fight all day. Felix Trinidad. Ran marathons in between fightts I believe. Carried his punching power late.
Henry Armstrong is probably the GOAT in that department in 15 round era. HM to Larry Gans, watch his Nelson fight. They are both insane after 20 rounds.