Maybe too obvious to be him (especially due to the William Blake poetry thingy), but I think out of the 7, he is most likely.
I think that Sophie Miller (Jane's doctor from psychiatric ward) is probably involved with RJ regarding Jane's old memory.
I can't see Partridge having the connections and power that RJ would have to have.
If RJ is Kirkland, why would he break into Jane's room for info when he had already rounded down the 7 suspects (as proved by Lorelei being alive (and don't give me any of that multiple recordings/cutting crap) before Jane got there himself. Also Kirkland seems waay to obvious, my theory is that maybe he has personal experience with RJ, wants his own revenge for something and wants to beat Jane or anyone else to it.
Stiles clearly has the ability to manipulate people in the way that RJ seems to, but in my eyes he doesn't have the right kind of personality. He is one of the most likely people I think, but again it may be too obvious, as his connection with RJ has not been hidden.
Haffner could be involved with Bertram and/or Stiles with regards to RJ, as Bertram assigned him to RJ case perhaps to find out exactly how much they knew. Or there could be some kind of visualize connection between Haffner and Stiles, if Stiles is RJ, Haffner may have been keeping him in the loop as Haffner is shown to be open to manipulation with visualize as proof.
I don't really have anything to say about Sheriff McAllister or Reede Smith, other than I would be disappointed if either of them turn out to be RJ, as I think it would be quite boring and I doubt the show's creators would choose anything so dull.
These have just been my ideas, and they show that I don't really have much of an idea of RJ's identity, but please comment if you have anything to question/add

I don't know who's RJ, but it's definitely not Bret Partridge. Everyone who thinks it is him is taking all the clues too literally. You can't trust anything of the details about height, weight, beard, hair, age, etc. since they change over time and could easily be frauded. 

Even if Red John is a group, there is still a leader to that group who is the true RJ. The 7 people list could be a white lie on the producer's part on so many levels. Maybe someone on the list is technically a RJ, but no the leader. Maybe Jane had more suspects and lied to Lisbon. 

Anyone arguing for PJ is also wrong. Multiple personality disorders do not work like that. There's no way he could keep a part of his personality away from himself and everyone else. Obviously the childhood memory is extracted from a relative of Jane's; there's no mindreading or hypnosis involved for that either.