I have narrowed my list to three suspects.  Let's keep in mind that Red John is probably in his 40's.  Red John is also under 6'0.  Red John was seen in season 3 finale. If you look at Red John he is not a heavy set man by any means.  He looks to be thin. 

Ron.  Ron could be Red John, but I just don't see him being Red John.  Ron could be the sleeper.  My gut tells me it's someone else.

Bob Kirkland.  Kirkland came out of the blue.  I don't see them having Kirkland as Red Joe.  There's signs that point to him being Red John, but Heller already knew who Red John was when they started it all.

Oscar Ardiles.  He would be the perfect Red John.  He could easily get a bunch of misfits to do his dirty work to get them out of jail time.  He knows the whos who of the CBI,FBI, ext.  And, as for a DA, and all the cases the CBI cracks, you never see the guy.  He has been there since the beginning.  He is a soft spoken guy, he's very smart, and we know nothing about his history.