In order to figure out who RedJohn is, you first have to figure out who RedJohn isn't.  Despite what apparently most people believe, it can't possibly be Jane for a couple reasons: 1) what would the point of joining the CBI team be?  There's no game there, and the one thing we REALLY know for sure about RJ is that he enjoys the game. and 2) the crew that created this show have made 4 and a half seasons so far of beautifully written, works of art.  Making Patrick Jane RJ ends the show.  And Bruno Heller said he will be revealing who RJ is in one of the next two seasons. 

It can't be anyone from Lisbon's team or Lisbon because they each have alibis for most of the RJ events that take place in real time during the show.  A lot of people think it's Van Pelt, but remember - Jane's wife and child were murdered more than 10 years ago, and RJ had already killed 8 women at that point.  Van Pelt is far too young.  Cho would be a lame RJ suspect so forget that one, and Rigsby has been in my head a few times, but I don't think he could play the big loveable dummy like that - i think he really is just a goofball.

Kristina Frye on the other hand is a very possible suspect.  A lot of people have made a very good point saying that she and Jane have a similar set of skills, which they do.  The only difference (AND THIS IS KEY) is that Kristina truly believes her powers to be real.  She honestly thinks she is psychic.  Red John and all of his disciples show no remorse for they're horrible crimes.  They honestly believe they aren't doing anything "wrong".  It's a very similar mindset.

Kristina mentions to Jane, in an attempt to prover her "psychicness" "you credit me with far too much cunning..." - trying to say that she's just psychic - not brilliant and manipulative.  But that is the point - she WANTS Jane to think she's just some crazy woman who thinks she's psychic when in truth she is brilliant and manipulative.  RedJohn mentions to Jane that he doesn't believe in an afterlife -  that he won't be punished for his "sins" on Earth.  Let's just assume for a minute that Kristina is RedJohn, wouldn't it be genius of her to convince Jane that RJ doesn't believe in the afterlife, and then whenever she's with him acts like a psychic who DOES believe in the afterlife?  It completely eliminates her as a suspect in Jane's mind.

Now of course there are only only 2 problems with it being a woman: 1) the fact that we've heard RJ's voice and.....eh.  It's hard to tell.  There is a video on Youtube that compares the voices and it makes a pretty good case for officer Brett Partridge.  But voices can be deceiving, and with technology it can be impossible to tell - remember the Scream movies?  Scream 2 one of the killers was the news reporter lady, but she had one of those voice changer things.  I know that those don't really exist, some can lower or raise the pitch of your voice but who knows how realistic they're going to be with this show.  And 2) Rosalind Harker slept with the suspected RedJohn under the alias name of Roy Tagliaferro.  Unless Kristina is packing something we don't know about, the only explanation is that Roy Tagliaferro isn't Red John.  He could be a disciple of RJ or he could just be a regular guy who knows nothing about RedJohn - but RedJohn knows him, and just uses his name.

So with the exception of the voice issue and the whole Rosalind Harker thing.  A lot of other arrows are pointing towards Kristina Frye.  But it'd be no fun if we figured it out now.  I honestly can't wait for Bruno to reveal who it is, I know for sure that they will or already have chosen someone great.

I really doubt that Jane is RJ just because I think writers/producers of this show are smarter than that.
Imagine how stupid would all the fans of this show (myself included) feel if they found out that the character they love and sympathize with is actually hartless and cold killer. I know I would be disappointed and feel kind of played because I cared for a character who is a monster.
That is why I doubt writers and producers would go that way.

Timothy Farragut.

He is the man that Bret Stilles allegedly killed (officially car crash caused by alcool) to wrestle control of Vizualise. Are we really sure he is really dead?

As the first guru of Vizualise, he must have many of the qualities Red John exhibits. He must be smart, well educated, good at reading people and a master manipulator. Pretty much like Red John.

He is also a cult-like figure, exactly what Red John appears to be in the eyes of his accomplices.

I expect him to have the dirt on Bret Stilles: he tried to kill him after all and killed Sheriff Elliot Elswit to cover his tracks. He is therefore blackmailing Stilles which explains the connection between Stilles and Red John and Red John's access to vast wealth (he offered Jane a whole new identity).

Robert Kirkland

 it is the man in the limo at the end of Red Dawn. His voice sounds the same as the man on the phone at the end of Season 4 when Wainwright is killed.  Notice how in episode 1 of this year they introduce Alexa Shultz, put her away for a few episodes, then bring her back for a throw back episode. Coincidence? Not likely. I just reviewed the first episode and notice how glib she is, how she seems to know that Jane is spinning a load of bullshit when he talks about a sign as a friend of RJ. She know's red john. Just because Heller has said that we might hace seen RJ, doesn't mean we have,but we probably have now and that man is Robert Kirkland. 

1. It's not Patrick. (Yes, it is really not Patrick)
2. It's not his father (RJ did this because he was "slandered in the media").
3. It's not a woman. (it should be obvious, but for example: RJ slept with Rosalind and I don't think she's lesbian)
4. We have already seen him.
5. A powerful and influential person.

So I'm going for Virgil Minelli and my secret favourite is Bret Stilles

In the end the truth will come and set us free