PFF's signature stats rates QB's on their standard grades but, on their signature stats they add deep passing, under pressure, play action, time to throw, sacks, drops, yards in the air, etc, etc, and a partridge in a pear tree. Their top 20:
1. Peyton
2. Rodgers
3. Rivers
4. Wilson
5. Brees
6. Cutler
7. Romo
8. Brady
9. Stafford
10. Big Ben
11. Ryan
12. TANNEHILL
13. Kaepernick
14. Glennon
15. Bradford
16. Newton
17. A Smith
18. RG III
19. Dalton
20. Palmer
21. Luck
I'm sure ther will be debate and discussion on this with many other stats brought in individually to prove a point one way or the other. However, the one thing PFF has done is to watch on film each and every one of Tannehill's 859 snaps this year, and his 1001 snaps last year ( he ranked #16). This is the eyeball test to an extreme, and takes into account who did what on each play when grading what RT did.
They also do the same for each QB listed above. There is a human element here but,I don't know of any computers with working eyeballs, and believe this to be pretty much accurate.