I have only watched the first season of the mentalist so don't know who to accuse. 
I think it would be really interesting if it turned out to be Patrick Jane with multiple personality disorder  or simelar condition) as I agree that Red John is always ahead of Jane.
However I do not believe that it could be Jane because we have heard him on the phone to Jane, laughing.  A low manly voice. ( can't remember what episode, in the first season after they found the couple who liked murdering red heads). So unless Jane made some one ring him and laugh at him just to throw himself of the track?.... doesn't seem likely.
Another reason why it couldn't be Jane is that Red John contacts Jane on the CBI network remember! And we know it was live as they attempted to trace it and it was followed until the 2nd base then re routed.  Jane was present on screen replying so that disagrees with the idea that he is Red John aswell.
I don't understand why some people think he is a woman, he is always described as a man and as i said the voice on the phone was male.
In my opinion, Red John is most likley to be someone who works at the station, someone who is seen continually all the way though the seasons but plays a really small part. Just enough so we cxan go back and say, "oh yeah, he was in that episode to hear about this".
Sorry if I am rembling. I can't wait to buy the second season!

Red John will be a character yet to be seen in the series... Although his accomplice(s) might already be known to au. This gives the writers creative plot room until the time when it Is appropriate to name the mastermind. Two reasons... Red John has influence in the FBI, which is higher up the food chain than the CBI whose characters we see. Second, when ared John is revealed, the plot tension goes, meaning the end of the series. Red John will be introduced to viewers in the final season.

  I believe that Red John is a combination, or cell of characters that all take a hold around Jane. The most likely (individuals) are Bret Stiles or Kristina Frye; Bret is a man with a God complex. (As pointed out in other "cases," he loves red, and runs a cult. He's screws with Jane at the worst possible times...
  Kristina Frye was his soon-to-be girlfriend before she was abducted. She was traumatized beyond speech... She was a "psychic" and told Jane that his daughter didn't suffer. This show is never going to allow an actual Supernatural factor, that would undermine the whole plot. She was there.
  As a "RJ Cult" Member, Grace Van Pelt *could* be part of it as reconnaissance. As pointed out in other cases, we know very little of her. Grace was new to the team soon-after Jane joined.
  Another theory (indirectly) points out that Jane could have severe MPD (Multiple Personality Disorder) and be Red John himself... I find this *last part* incredulous because there are no seemingly long absences between them. However, it could be compared to "Fight Club," the leader of Fight Club was regarded as a demi-god.. He didn't know it until the climax of the movie.
  Red John may be attempting to drive Patrick Jane mad, so he could take over the "Jane body." His Cult would follow him blindly and do what they could to drive Jane mad... I think this is the most likely twist, one person is not enough for such a masterpiece, it has to be at least three people... 
 Please speak of any points and counter-points that come to mind! Thank you for reading!

Red John is probably yet to appear on screen, but he has many people working for him (willingly and unwillingly) and therefore we feel like we have seen him. All the clues are there for a reason.

Would't it be horribly scary if many of the side-characters we have met were infact working for Red John (like a godfather operation) and little by little, Jane finds himself surrounded by Red John accomplices - and has been for a long time? Yikes!!

The writers will have a hard time explaining why Jane is so important to Red John, but this is NOT "Lost" so if any of the writers are reading this:

*****Red John CAN'T be Jane*****

It would just be wrong and not in conjunktion with the spirit of the show. 
It really shouldn't be Cho, Van Pelt, Rigsby or Lisbon at all, either.

A business man who's life got destroyed by Jane from his psycic days, would be most believable and more scary due to the calculating, ice cold. Maybe Van Pelts father, that's a good idea - IF Van Pelt is unaware (or at least don't want to admitt to herself) of it.

In my world, Red John hasn't killed one victim himself.
He lets other people do it for him.
That's why Jane has to kill him - because there is NO way the murders can be connected to Red John legally.

It would be exiting and refreshing to see some seasons ahead, after the big confrontation, where Jane becomes a happier person. Many shows does reveal the big thing (early) and gets away with it. Just change the direction, keep the people. 

No reason why the Mentalist shouldn't be able to have a life after we find out who Red John is.
:)

Red John has cult following him, like Bret Stiles and his accompliances. Brett Partridge is one of them, like Craig O'Lauglin was. It's no one from his team, because that would be just too weird for mainstream tv show and Kristina Frye has now some mental problems and that disqualifies her. Also  I don't believe in RJ=PJ theory, just because that would be too big twist.