Best stamina in Boxing..

[walrus]

Butterbean#1

[ruthless rocco]

Johnny Tapia

[Dark Lord Al]

Johnny Tapia

Good shout.

[mikeeod]

In no particular order:
Hopkins
Hagler
JC Superstar
Troy Dorsey
Wayne McCollough
Barrera
Morales
Margarito
Zack Padilla
Ricky Hatton
Floyd

As for exercises that increase stamina, there are many: jump rope, running, sparring, bag work, cardio equipment (bikes, rowers...etc.). I've found that certain natural/physiological/genetic factors effect stamina levels. Some people have larger lungs, higher VO2 Max, more efficient hearts...etc. When these individuals also have high pain tolerance, mental toughness, and discipline, they end up as the best conditioned in their sport. No single exercise is responsible for great stamina though.