I don't have an answer, but just wanted to say that fast defense-first boxers are the toughest to beat because they're not looking to KO you, and thus really don't put themselves at risk. Did anyone really beat pernell in his prime? I guess trinidad eventually got to him (although he was past his prime) so the answer probably lies in a fighter with a powerful, fast, and precise jab mated to a long arm...not unlike the way hearns was able to handle leonard.

i've seen floyd have some trouble with guys who pressure him and get very close, but no one has been able to put him over the edge.