Muhammad Ali and Rocky Marciano come to mind easily. Marciano was never stopped while Ali was stopped only in his comeback fight at an advance age against the younger and very prime Larry Holmes. Sugar Ray Robinson, too was stopped only once in 200 fights and that was because he succumbed to heat and exhaustion in his failed bid to snatch the light heavyweight title from Joey Maxim.

Joe Calzaghe and Evander Holyfield

a few..... lamotta, jeffries and the human punchbag joe grim all come to mind.

edit;

can't forget ali or marciano.

add joe frazier to and larry homes to that list