It could really be anyone which is the infuriating/addictive thing.  of the 7 on the list Ray Haffner ticks the most boxes for me

-similar to Harkers description
-member of visualize and right age to draw the face on the barn
-connected with Stiles as above
-he  is introduced as making his career off surveilance cases. Think the wire tap in the girls room at the end of the first series. Red John constantly seems to be a step ahead. Surveillance skills would surely help here
-was a member of the FBI and CBI

There are other guys i would say are right up there. Ellis Mars is one. Minelli another and Sheriff McAllisters inclusion on the list puzzles and intrigues me. If i had to pick one person i would say its Mars. If the list is right, Haffner.