James toney or our era, but Floyd is a close second, like him or not, and I don't like him personally, all time? Probably Archie Moore. Toney got his style from Charles, Walcott and Moore. He said so himself, but even though all those fighters were sneaky, the only pure counter puncher was Moore. I would have put Hopkins in here but again he was sneaky offensively more than a pure counter puncher.
Duran himself was a very skilled counter-puncher, along with Benitez, as has been mentioned. Sal Sanchez was another who excelled in this area. Napoles, Whitaker, etc.
almost every fighter tries to counterpunch.. but as apure counterpuncher.. im not too sure.. Marquez is a good counterpuncher
I'll throw in Locche, as that was pretty much his only means of offense(and boy did he master it, especially in close, master of the philly shell) aside from his jab and lead left hook.