Duncan is top 5 all-time IMO. He is constantly doubled on offense which takes away his numbers, but he's such a great passer that he sets up his teammates out of the double and there's no stats for that.
On defense, he may be top 5 for his position all-time as well. The numbers are solid but once again, the stats don't show how smart he is when playing D. His traps and help D stuff he does that many don't notice is picture perfect on almost every play.
I've loved this guy ever since he was at Wake and sent me two autographed photos. Guy was in college and still paid for the photos and shipping and whatnot. I thought it was the coolest thing ever when I was growing up. All that Peyton Manning bastard sent me was a facsimile autographed photo.