Since PFF has recorded stats/made them available, Watt graded out to have BY FAR the best season tracked. He finished with an absurd +101.7 grade (55.2 points above 2nd place for 3-4 DE). As a bit of scale/perspective, a pretty damn good season from Justin Smith graded at +22.2.
Linksauce to his Weekly Grades
Also of note, Von Miller's grade of +78.5 is the second highest grade PFF has ever given. It's a whole 61.6 points above the 2nd place 4-3 OLB.