What is your definition of “best” if you are asking about older iPads to run the iOS version of Photoshop on?
if you are looking for the best iPad, the best iPad to run Adobe Photoshop for iOS on is the current, 2018 , 11 or 12.9 inch screen iPad Pro w/2nd gen Apple Pencil.
Even the cheaper 10.5 inch screen, 2019 iPad Air 3 should run Photoshop for iOS very well, also.
The 2019 iPad Air 3 uses the older, 1st generation Apple Pencil, but it is still very responsive on newer, non-Pro iPads.
For you to determine your needs, you really need to go to an Apple Store or authorised Apple product reseller and check all of these iPad models out to make the “best” decision for YOU!
Best of Luck to You!