I think a better question is: "Is Duncan the best PF of all time?"
I know people will overwhelmingly call him the best PF of all time, especially since he is playing now. But the saying "out of sight, out of mind" is so true. I think its an extremely close race between Duncan, Malone, and Barkley. Malone makes his case with extreme consistency and underrated defense. I'd give Duncan the edge though due to being a better post player, both offensively and defensively. Barkley, though he will average 3rd in most surveys I think, is actually a tougher one for me. People forget how freakish this guy was and how dominating he was. Duncan has always had the benefit of playing with other great players (and more than that, D-rob one of the best defensive Cs of all time. If Duncan was drafted by any other team, he would likely be a C and not a PF. In fact it was a genius move by Pop cuz D-rob was such a great defensive C and Duncan always had trouble with Shaq on both ends of the floor when matched up against Shaq. Thus he gets to be the best PF at the time instead of the 2nd best C of the time.), but Barkley was alone for most of his career and unstoppable. (And he'd have a ring if not for a guy named Jordan. Barkley played ridiculously well in the Finals...unfortunately, Jordan played out of this world ball in the series too.) Obviously Duncan is the "greater" player due to his rings...but a "better" player I'm not so sure.
Thats just my opinion....but I think most would say he's the best PF of all time.