I remember a quote on Chuvalo about if every fight went a 100 rounds he'd win every single one of them. Marciano and Jeffries were both iron men who seemingly never tired and were both capable of late, one shot KO's. Cotto comes to mind with current fighters. Not an ATG great or anything but his entire career is filled with late knockouts, including more than a few 11 round ones.
Juan Diaz and Paul Williams. Neither one of them has a lot of power but with all of the punches they throw, it's got to be like fighting a swarm of bees.
I look at the guys who fought 20 and 100 rounds and they slow it down. I look at guys who could go 15 hard and I think they sure had more rounds in em. Those guys would probably be the fittest. I'm think Hagler, Pryor, Armstrong, even up at heavy we got Frazier and Marciano (though they are slightly overrated).
The hook that ABC used to attract viewers to the Holmes/Cobb match was the suggestion that if every fight was scheduled for 45 rounds, Tex might never get beaten. The temperature was well above 100 degrees in Havana, when Willard took the HW Title from Johnson, and he looked like he was good to go for the scheduled 45 round distance. The Corbett/Jackson fight went 61 three minute rounds on a barge under a blazing sun. This was before TB ravaged Jackson, and booze began dissipating Corbett. This 61 round draw was also the first bout in which an automatic timekeeper was used. Aaron Pryor's 14th round performance in the first Arguello match was something other than human, foreign substance in Panama Lewis's water bottle or not. Ted (Kid) Lewis won 19 20 round decisions in his career, a 19th round KO, an 18 round decision, and two 17th round kayos. Scoring a kayo beyond round 15 under the Marquess of Queensbury Rules is a rare and remarkable event. Generally, when a match scheduled for longer than 15 rounds went beyond that mark, it tended to go the distance, whether that limit was 20 or 25 rounds. Now, according to my 1957 copy of The Ring Record Book and Boxing Encyclopedia, Andy Bowen drew against Jack Burke on April 6, 1893, in the longest glove fight on record: 110 rounds, 7 hours and 19 minutes. (The referee called it off when neither could continue.) South Africa, 1890. Barney Malone W 212 rounds Jim Silberbauer. Time not listed. The last scheduled 25 round fight was between Bill Poland and Eddie Blunt, on August 27, 1941 in Washington, D. C. (Poland kayoed Blunt in four rounds.) The last scheduled 20 round fight was in Boise, Idaho. On June 9, 1954, Bobby Woods knocked out Henry Davis in 15 rounds. Max Baer was the winner of the final match to go the 20 round distance, when he dominated King Levinski over 20 rounds in Reno, under the promotional banner of Jack Dempsey. (Baer may or may not have hit as hard as Shavers, but there's no question who had better endurance.)