This is a tough question because a lot of players from the early years were more "big men" (i.e. PF/C) than strictly "power forwards" or "centers." For example, it's tough to say how to classify Wes Unseld, Elvin Hayes, Moses Malone, or even Bill Russell. Even Tim Duncan kind of blurs the line between PF and C.
That said, here's my ranking:
1. Tim Duncan
2. Karl Malone
3. Bob Pettit
4. Charles Barkley
5. Kevin Garnett
6. Dirk Nowitzki
7. Dennis Rodman
8. Kevin McHale
9. Jerry Lucas
10. Cedric Maxwell