Red John has short straight hair (as described by Rosalind Harker) so BERTRAM and STILES are out of the picture.

Red John has to have a good physique and strong so Reede Smith is gone.

Now left with Bob , Haffner and McAllister.

According to Dr . Miller  Red John is a very good whistler ,  Out of the three options left in s06e02 they showed Haffner and McAllister whistling just to mess with us. So most likely RJ would be BOB KIRKLAND !!!!!!!

And lastly Kirkland's voice is almost similar to RJ . He is the only one with an evil look in his eyes...

Just an hunch..