Hall of Fame
Doesn't matter because everybody is biased.PFF grading has come a long way since it formally began player grading in 2006............it now is teamed with scouts, coaches and former players. BUT I'm going to give you the last word.........because otherwise this nonsense will have no end.
There are so many factors you can't account for.
The Titans with Tannerhill might be more efficient than with Mariota.
A pass rusher that plays against a gympy Mahomes has a better chance with him than a guy who plays agaisnt him in full health.
A number of HCs (Dungy, Gruden) had said that there are often times we don't know for sure about the protection call, how a receiver adjust his route (each team can have their own philosophy and it may even change from year to year).
Grading a unit, or the whole defense, or the whole team is more accurate , but may still be incorrect.
Some teams may play in a snow storm, a freezing day, a heavy rainy day, a windy day, etc.
It's not like in a lab where you can keep "everything else being equal".