Just wanted to say really enjoyed a lot of the theories so far, some really well thought out, intricate and very plausible ones. Not so much a theory as to who exactly Red John is, but my top three suspects (in no particular order)

1. Gale Bertram (Director of CBI) 
2. Brett Partridge.(Forensic expert at CBI)
3. Ray Haffner. (ex-CBI agent and known connection to visualize)

I choose these three as they have direct access to CBI, even though the others have had access too. 

Other possibilities.

4. Bret Stiles. 
5. Sheriff Tom McAllister.   

I have chosen to eliminate both Bob Kirkland and Reede Smith. Kirkland because I think it's too obvious it's him and that Heller is playing a bluff (breaking into Jane's "office" and stealing his files and findings into Red John) and killing RJ's accomplice in the hospital ("Do you know who I am"). I've eliminated Reede Smith purely on instinct, I have no reason why, but I just strongly feel he is not Red John.