I have read with amusement many of the theories, and kudos to all who take part, since we share in a common passion and pursuit. But no doubt a year from now, when Red John is known by all, someone will troll back through the postings to see who had it right all along, and that will be .....ME ....hopefully. I've purposely waited before posting, wanting to research all the possibilities before weighing in. Here's the process I went through.
FIRST - we know that Red John has had many associates, and some if not all of them appear to have an almost fanatical devotion to him. How? How could seemingly reasonable people in every other aspect of their life give up their own logic for the murderous logic of Red John? The answer is -mind control. We've seen Jane do it himself (Jane is not red john) and we've seen Rigsby under hynosis, as well as the Russett potato guy and several others, including Christina Frye (Christina is not red john). Christina Frye is a therapist, well versed in the ways of mind control, and yet -she has been mentally overcome by her experience with Red John. SO, RED JOHN is a master hynoptist, just like Jane is.
TWO - If you go back through the episodes and look at all the therapists, hypnotists, and mind-control people, eliminate the women (red john is not a woman), you are left with only 4 people.
THREE - The telling episode which gives away the answer is "Red Moon" the first episode that Simon Baker directed by the way. At the end of the episode, Todd, the killer, has a message for Jane. But before Jane can get there, Todd is set ablaze, and then as dying, quotes the Tyger, tyger poem that Red John uses.

So why would Todd refuse to kill the patsy that Jane accused (falsley on purpose) of being the murderer? He had already killed 3 people to cover up his tracks, so why stop now? The answer? The patsy was RED JOHN! That's why Todd wouldn't kill him, even at the risk of implicating himself. And who was the patsy? Uh, just a guy named ELLIS MARS (remember the episode with ..."...HE IS MAR..." ? And who is Ellis Mars? He's a fake mentalist (supposedly), someone who predicted another death would occur after the original three. How could he know this unless he was either the murderer or controlling the murderer? In addition, he speaks in "polite speak" just like red john did in the warehouse "Christina would want me to send her love. She's very fond of you, despite your differences." This type of formal English isn't used by Brett Partridge, it's used by ELLIS MARS. Plus, Ellis Mars gives Jane a business card with a tiger on it, which we see in the next scene as Jane holding it up to the ceiling while lying on his bed and reciting the words "tiger, tiger". Ellis Mars fits the speech, the age, the physical appearance, and the set of skills necessary to pull it all off avoid suspicion. The reason that no one considers him is that he comes across as bumbling, a fake, a wannabe, and he's easy for us (and Jane) to dismiss. But this is exactly what RED JOHN WOULD do -he wouldn't come in wearing a black cape and long moustache, he'd interject himself quietly and unobtrusively into the scene (think of The Usual Suspects - Kaiser Sosay). Ellis/Red John is playing a game, using his mentalism as a trick to gain entry, just as Jane does. Remember when Lorelei told him that he and Red John are similar? And the name Ellis (not my discovery) can be loosely translated in spanish as "Ellis = "El es"= "He is" MARS. It's just that we can't see it because it's right in front of us. 

So that's theory, and I can't wait for our Alien overlords to review this entry in 50 years to realize I was right, and hopefully spare me from constructing their temple to the Gods...

Best of luck to all in your search....