Both good guesses! Of the two, Rosenbloom had the longest run (185) without being ko'd... which isn't enough to be the answer to this question.
BINGO! Miller had 252 pro bouts, and was only ko'd in his 2nd, and then years later in his very last fight. So in between he had a streak of 249 fights, where he wasn't stopped inside the distance. Well done :good