All time is impossible to rate because nobody has seen every era, let alone enough of it. Not to mention that athletes get bigger, stronger, faster as mankind advances. It would be unfair to put Lennox Lewis against the 187 pound Marciano. So going on the eras I have seen plenty of it is this in any order.
Pernell Whitaker
Roy Jones Jr.
Bernard Hopkins
Ray Leonard
Floyd Mayweather Jr.