He smoked 2 or more packs a day (1 a day when training) didn't kill himself doing roadwork, didn't engage in ring wars when sparring, and was "slow and stiff", but he was never knocked out, was pretty much unmarked after his career was over, and reversed the only losses in his career (3) and retired as unbeaten middleweight champion.
Margarito deserves mention here. He's basically a junior middleweight at the least at his "real" weight, and he's got impressive stamina for a junior middleweight that size.
I think Bat Nelson deserves the honours. The guy was inhuman. He averaged 85 punches per round over the course of a 42 round fight!!!!!!!!!!!!!!!!!!! Tat would be a prety good workrate for a 12 round fight.
Henry Armstrong immediately comes to mind. Apparently the physical size of his heart was larger than the average person for his size. Joe Frazier also had freakish stamina. He always came forward, bobbing and weaving while throwing punches. Dr. Ferdie Pacheco on the "Greatest Fights Ever" broadcast of the FOTC, said Joe Frazier had the greatest quantity of stamina. Honourable mentions, Archie Moore, Evander Holyfield, Aaron Pryor.
The man won the title at 39, was denied for many years and even during his early 40's displayed pretty good stamina. His fights against Harold Johnson, Joey Maxim and Yvon Durelle speak for themselves. Archie probably doesn't score well on the stamina scale in comparison to others fighters known primarily for endurance, but the fact that his KO power barely diminished in the late rounds speaks for his stamina.