If Duncan is not the #1 forward in the game, the only possible compe ion is Dirk.
I agree with others that for the career its unquestionably Duncan--even if you spot Dirk 3 years because he came into the league as basically a European high-schooler while Duncan came in a year earlier as a polished college product.
During this season, with Duncan's productivity down due to the plantar fasciatis, Dirk was clearly the better player, but in our series I thought it pretty much a push with Dirk and Duncan both playing at an MVP-level that basically cancelled the other out and Dirk's supporting cast was slightly better.
It will be interesting to revisit this next year when Duncan will presumably return to last year's, pre-PF form. Duncan is still better than Dirk at the 'classic' power forward aspects of the game, down on the blocks both offensively and defensively. But Dirk is more versatile with better ball-handling skills and a better shooting touch from pretty much anywhere on the court.