Hands down an all time great. But Karl Malone WAS a PF. Duncan is a center, lets be honest. he just came along at the right time and played alongside Robinson. he will always be referred to as a PF, but he truly was a C. I guess I would hold them both in equal regard as the best PF's to play the game.
Its kind of hard for our generation, and really unfair that we didn't get to see some of the old timers play in their primes. PF's like Bob Pettit, who defined the power forward position, McHale, and Barkley(who I only got to see in the twilight of his career) are all guys you could make a strong case for as well in the PF discussion.
in closing, to answer your question Claudius, duncan is definitely the most under appreciated player of our generation