I don't really have a theory on who Red John is because a lot of the facts we pick up from the episodes could just be garbage, the characters could be lying.
Or worse based on the suspects here we have to chose from , how could someone like Jane not figure out who he is, having met him and all(or shook hands). From this I gather the identity of Red John is either going to be really surprising or a complete disappointment.
If it's any of the main cast that person is most likely going to be a pathological liar with their psychotic ways under control, and can obviously fool Jane without difficulty. I've had experience with these types of people and in my unprofessional opinion his best chance to find out who RJ is, is to get him alone, one on one.
