Some interesting theories floating about here, here are some of mine:

1) It is NOT Brett Patridge, he seems weird but is not even close to having the influence, power, connections to be Red John
2) Brett Stiles would be a good fit but I think he is too old (also he has always wanted to help Jane e.g. finding Kristina Fye and getting Lorelei Martins out of jail)
3) It is not Ellis Mars or CBI Ron (same reason as Brett Patridge as they do not have enough power and prestige and Ellis Mars seems really stupid and was easily exposed as being a fake psychic)
4) It is not Walter Mashburn, he does have power and influence but i think he lacks the mental capacity and evilness for being Red John.
5) Dean Harken could be a good fit as he has power and influence and also seems to be smart but he was easily fooled by jane's plan in the bio-chemicals episode so I am not sure.
6) Director Bertrand and Virgil Minelli have always seemed kinda mysterious to me and not what they appear to be. I think they are NOT Red John as they do not match the physical description and age of him but I believe they may be accomplices or somehow connected to him.
7) Bob Kirkland is really popular as a choice to be red john among followers of the show but I do not think it is him, it would be too obvious although I do find it weird why he is obsessed with Jane and red john so i think he may be an accomplice trying to cover red johns tracks as he may suspect jane is getting close to figuring out who red john is (he might have gotten jane's research to study it himself and see if red john has made any "mistakes", clues or trends which he would then tell red john about to not repeat it)
8) Ray Haffner could also a good fit for red john as he is a senior member of the CBI and is quite smart himself (starting his own private agency). I honestly do not know too much about him to make a prediction
9) Van Pelt has also seemd mysterious to me from the start, Jane mentioned that she suffered a past trauma as a child and her sister commited "suicide". I think there is more to her than what the show has already shown. I do NOT think she is red john but she could be a long term undercover accomplice or something.
10) Oscar Ardilles has always seemed antagonistic to me so he could be Red John, he is resourceful and seems to be intelligent. Plus his name O Ardilles is an anagram for "a soldier" and Lorelei mentioned that Red John sees jane as "comrade". Also, "Cut Iron" is an anagram for "in court"
11) It is NOT Patrick Jane's father either, he is smart (taught jane everything he knows) but probably is not resourceful enough and would not ruin his son's life by killing his own granddaughter and daughter in-law. Plus he would be too old
12) One interesting theory I have is that it could be Jane's twin brother or something that even jane does not know exist (in the show, NOTHING has ever been mentioned about Jane's mother). The twin brother could have become bitter about Jane's success as a psychic and for be estranged so he exacted revenge on him by killing jane's wife and daughter and being a serial killer. This would explain his intellect.

In summary, I think the best bet for Red John could be Ray Haffner, Oscar Ardilles or someone from Janes past carnival life like a twin brother.

Thoughts?

IT IS NOT KRIKLAND!

That's all I have to say, and I Like Walter, that's it.

PS:
And it is not Patrick, who the hell believes this!?

Greetings,
Dexter

First of all, i love The Mentalist but the idea of a serial killer with a network is completely ludacris.
Organized crime bosses and serial killers have two very different types of personality ...
I think that the theory of jane being red john is funny but impossible for many reasons described before, mainly because most of the people in red john's network that met jane didn't face him as if he was red john, specially rosalin harker, timothy carter and lorelai martins.
I believe that red john could be one of three persons ... Brett styles (even though i think he is too old for the character), brett partridge (in this case i think he might be too young) ... And kirkland ...
Kirkland is old enough, has power, means, money, connections ... He killed a guy who could identify red john on 5x18 ... He seems very aware of everything and seems to know jane pretty well but jane doesn't know him at all ... Besides ... the theory that he couldn't be red john because of lorelai being in prison at the time he and jane shook hands, doesn't convince me, i believe that he had met and shook hands with jane before that instance ...
Other theory is that Kirkland is another of red john's friends ... And in that case Ray haffner seems a good suspect ... He might have been in the  visualize farm when the red john face was painted ...
He is a member of visualize, and that gives him some kind of power and connections ... besides ... styles might be another of his friends ...

If you watch the "Red John's Footsteps," notice how to tea cup that Jane, Harker, and Lisbon are drinking out of is the exact same tea cup that is shown in the scene where RJ has apparently fled the house where Jane is being held at gun point.

Now, it could be that RJ stole the tea cup from Harker's house, but what if it's a subtle clue from the writers the RJ is, in fact, Harker. If RJ is a man, that's an awfully feminine cup for him to be using. But hey, to each his or her own;)

We are never told where Harker is during the showdown at the country house with Sheriff Hardy. She could have easily been there waiting for Jane. And it would be brilliant of RJ--as Harker--to throw everybody totally off the traill by describing RJ as a male with all the other characteristics she listed, when it is really her all along.

And, being a blind person--with all her other senses heightened like touch and feel--how wouldn't she have been able to smell the scent of fresh paint above her bed after RJ painted the trademark smiley face? It isn't blood above the bed--no killings had taken place at her home at this point (and the post-killing smiley faces are larger and dripping down the wall). It's rather odd, however, how Jane doesn't mention it to Harker. Another thing to point out above the smiley face is that, if Harker is really RJ, they both have smiley faces above their beds--well, at least Jane did while he still lived at his house where his family was murdered.

I personally think it would be awesome if it was Harker, with Jane sitting listening to her playing piano in the episode, seated right next to the person he always searching for. Perhaps I am wrong, but something about the tea cup seems a little off to me. It's a nice subtle hint, I think. But perhaps I'm making something out of nothing. It's the exact same type of tea cup, though. It's odd that Jane doesn't pick up on this because he notices everything.