In terms of, who would beat all others at 140... Id say Whittaker, followed by Benetiz, Pryor, then Chavez... I know he never fought at 140 but I think Shane Mosley could have potentially beaten anyone on the list at his best. Also if Floyd was on your list I would have him at 2nd or 3rd.

Whittaker
Mosley/Floyd
Benitez
Pryor

MY top 5.
Chavez has a great record/legacy, probably had the most signifigant reign at 140, but I don't think he would have beaten one of those fighters at their best.