Floyd Mayweather jr. and Pernell "sweet pea" Whitaker are two fighters who both have the words "the greatest defensive fighter of all time" attached to their names.
Who do you think is better and why?
I've never liked Floyd as a person, but as a boxer, i think he is brilliant defensively. By that same token, Sweet Pea was also amazing defensively. However a direct comparison is rather hard to achieve in my mind.
Pernell moves better, his ability to create angles and slip punches is second to none, and I don't think he was ever seriously hurt in his prime, even against some very game opponents.
Conversely, FMJ utilizes the shoulder roll and parry without wasting much energy and creates excellent counter punching opportunities, but he has been seriously hurt a number of times.