Guys with the best chances (not surefire winners, but guys who would give him a run)....

Tommy Hearns
Bob Foster
Michael Spinks
Dwight Qawi
Gerald McClellan
Julian Jackson

Really super short list and I'm sure I left out some people but RJJ in his prime was amazing. Defensively akin to a young Muhammad Ali, didn't even really have to block shots, he just avoided getting hit but he had insane combination handspeed but also with 1 punch KO power.

RJJ is one of the few guys who could score a flash knockdown with handspeed alone but also just turn the lights out on you with his power...dangerous boxer in his prime