The fighters that spring to mind are ..

Frank Tate.. I once spoke to Manny Steward and he told me that this guy trained like you threatened him with a gun...
Tony Tucker.. One heavyweight who really pushed himself to the limit for everyfight before the Tyson bout...It kinda broke his spirit..

Ruslan Chagaev.
First to Gym, Last to leave. (Kenny Adams asked him to do something and he did it until you told him not to do it)

Vassily Jirov..... He trained like a monster.. he even looked posessed...
Merqui Sosa.. He told me that the more pain he felt in training , the less pain you will feel in the fight...

Tim Witherspoon.. (just joking)