In my (limited) experience, Aaron Pryor is just one guy who stood out when it came to his conditioning. It was just amazing. I would imagine the guy would be a pressure-type. Is there one guy you feel that really was in a class by himself??
Harry Greb is my number 1 easily. Battling Nelson and Ad Wolgast are 2A and 2B on my list. For the modern era Pryor is up there. So are Troy Dorsey, Freddie Roach and Tommy Cordova.
Henry Armstrong, no contest. Marciano was excellent for a heavyweight. Greb the human windmill must have been good. Tunney was also very good, given how much he moved. Pryor trained for 30 round fights and it showed. Qawi had great late-fight stamina. Salvador Sanchez could keep going and going. Calzaghe had exceptional conditioning and punch output even in the later rounds, but he only fought in an era of 12 rounders. Trinidad had great conditioning too.
hands down jeff harding easily. He didnt even train 4 mccallum and didnt look tired once he partied right up till that fight
This is a good list although guys from the 12 round era are just hard to compare against the others. But Pacquiao and Calzaghe do stand out. Fenech is a good mention.