I mean Duncan is a better player overall but he was never an amazing offensive player. Steady and consistently very good but never super elite. His all-time level D and great rebounding gives him the overall edge but as far as pure offensive I think its pretty easily Harden. I'd definitely put Dirk over Duncan when it comes to pure offense.

But yeah overall it's difficult, Harden is pretty unique in his scoring+efficiency+playmaking. Bird and LeBron are two of the only ones who come to mind as far as that goes. Only Kareem, Dantley, Jordan, Mailman, Durant, and Curry have ever had a single season of 30+ ppg and .600 ts - Harden is very likely to do it over a full three year span, something only Dantley and Jordan did. Then throw in his playmaking...

I dunno, cross generation comparisons are just hard in general. For this current generation I think its definitely LeBron/Curry/Durant/Harden in some order. Beyond that its just wild speculation, which can be fun, but mostly just leads to bitter arguments online :P