Obviously Robert Bob Kirkland.
Whenever a case comes to CBI which is about Red John, Kirkland appears there suddenly and the case goes directly to the Inland Security.
You can watch on YouTube ( v=L5-H9QGov4Y ) Patrick Jane and Robert Bob Kirkland first meeting video that their first shaking hands as Lorelei mentioned about it.
My theory is Bob Robert Kirkland is obviously Red John.
