I could see him 2 behind Duncan. Only because Duncan's success. Talent wise there is probably no one better than Giannis unless you want to count LeBron or KD as PFs playing SFs.
Giannis is the only PF in history that has at least 2 MVP's, 1 DPOY, and 1 Finals MVP. KG is the only other one with at least 1 MVP and 1 DPOY and Duncan and Dirk are the only other ones with at least 1 MVP and 1 Finals MVP (TD has 2 MVP and 3 Finals MVP, Dirk 1 each). Pettit also has 2 MVP's and almost certainly would have won the 58 Finals MVP had it existed then. The Mailman (2), Dirk (1), and Sir Charles (1) are the only other PF's with a MVP award. Rodman (2) and Draymond (1) are the only other PF's with a DPOY (and no one before 83 really stands out except for maybe Hayes and Bobby Jones as having a shot at one before they started awarding it). Hayes should probably in the discussion as all time great PF's as he was an elite defender and absolutely should have been Finals MVP, they gave it to Unseld as almost a consolation, but Hayes outrebounded him, more than doubled his scoring, out stole and out blocked Wes. Basically the only think Unseld did better was pass the ball and Wes didn't foul out of game 7 (which I think is what gave Wes the award).
Giannis is only 8 years in, so lots of time to go, but he is certainly trending like he will end up as either the 1st or 2nd (to Duncan) best PF of all time.
Since Giannis is still playing, I won't include him in my rankings, but I think if I were ranking the retired PF's, I have something like this for my top 10 (outside of TD I could potentially be persuaded to alter the rankings, though it would be pretty hard to move Mailman off of 2).
TD
Mailman
Dirk
KG
Charles
Hayes
Pettit
McHale
Rodman
Gasol
I thought a bit on 10 ultimately deciding on Pau as a lot of his best years were at PF. I think Schayes and Lucas would also have arguments among the retired players (or at least players where more time won't change anything so basically no Giannis or Davis).
If Giannis didn't play another game, I'd probably have him between Charles and Hayes and if Davis retired tomorrow, I'd probably have him ahead of Pau and maybe Rodman.