I think it is Sheriff Thomas McAllister. He is only the Red John suspect who's age has been pulled down from his real life age. He is really 57 but in the show he is 50, it seems like they are trying to make him younger to fit to be Red John. Why didn't they change other characters ages? Why only him? 

He is also the most unlikely suspect that we think he has no possible reason to be Red John. Just like in all of the episodes of The Mentalist, we only know reasons of the suspects to kill whoever has been murdered and we only find out the reason of the actual murderer when he explains it when he gets caught by Jane.

He and Ray Haffner are the only suspects out of the seven who has appeared with the word ''Red'' on the title of the episode. Also the case of the married couple in S1e2 in Napa connects to the story of married couple Timothy Carter and Sally Carter. And on the next episode of Mentalist they are going to focus on Thomas McAllister and things will happen that has no connection with Red John but I think that the things that will happen will make sense when they slowly get closer to Red John.

based on the he is MAR thing it can only be

Gale bertram because bertram in reverse is MARtreb
 or 
MacAllisteR

Bertram is like 6'3, is almost bald and i can't really see him beeing RJ. so my pick is the sheriff

 Hi guys, i will try to look in the small details, cause for me if we look in the bigger picture, it's possible to accuse any of the suspects, Bruno Heller is playing with us, not a very nice guy if you ask me.

 I'd like you to keep some things about Red John in mind while reading this.

1. He admires Jane, he thinks of Jane as a 'friend' and even loves Jane in a certain way (Brett Styles says that, and he kept and saved Jane's life until now for a reason).
2. He's bold, not afraid of being caught, he knows how damn good he's, so I don't think Red John wouldn't do extreme changes in his personality just to keep the police off his tracks.
3. He loves women, in his own psycho and disturbed mind, but he do.

Ok, now with the suspects, i will stay with the 7 suspects, cause I don't want to go mad thinking about all the others.

Partridge: Dead. Period.
.. Okay, even if he wasn't, if you think about the 2 point I listed above, Partridge plays dumb, don't think that a narcissist would play a dumb character just to avoid suspicion. And yet, Partridge hates Jane.

Bertram: like this one, and till the S06E02 he was my favorite, but some things happened.
Assuming from season 3 finale that he's connected to RJ, or is RJ himself (a lot of theories about this on this site, not going in details, sorry). Now in the reunion with Reede Smith and Mcallister, Reede Smith was literally pushing Bertram, giving him orders and saying what he's suppose to do. If Bertram is RJ, it would not happen, not with Reede Smith leaving the room alive.
Oh, and Rigsby thinks he's RJ, Rigsby is always wrong.

Brett Styles: hard to eliminate too, but in one of his conversations with Jane, he talks about RJ as someone bellow him "Oh, I know things that you and your friend Red John would never imagine" or something like that, I don't think he would put his alter ego in a position like that, it's like Bruce Wayne badmouthing Batman, and again, he's a narcissist, a narcissist never talks this way about himself.

Haffner: don't remember the episode, but when he assumes the team, Van Pelt tries to talk to him about some development in the case, he ignores her, not even a glance at the beautiful Van Pelt, RJ is obsessed with women, it goes against his nature to ignore beautiful women.
   Oh and about the Lisbon not being dead that some people are arguing that RJ didn't kill her because he likes her.. I don't think so, Red John asks Jane to kill Lisbon. Ok, he knew that Jane wouldn't do that, but only the asking part is good enough to assume that RJ doesn't care that much about her.
   Now, if you ask me why RJ didn't kill Lisbon, I think that is because he knows that if he killed her, Jane would have another break down and spoiled all the fun he has 'playing' with Jane.
  Another aspect about RJ here, he likes to play his mental games with Jane, and I know all of you already noticed this. With Lisbon dead he would miss this. So I think RJ is leaving Lisbon for his grand finale.

 Bob Kirkland: Don't have much to say about him, except that he does things in a way too.. rusty i guess, hiring people to break Jane office, killing the guy in the hospital with a pillow, seems too much no-red john to me.

Reede Smith: again, not much, he indeed gives orders to Bertram, but he seems a little bit uncomfortable (body language) in the meeting in this episode. And he's too fat, but I'm trying to stay out of physical characteristcs here.

McAllister: In contrast with Haffner, do you remember what was his reaction when he was alone with Van Pelt? Creepy, a lot creepy. In the meeting scene of the last episode, he was the one with the more relaxed body language, like "I own this place", he didn't talk much too, and very often the person who talks less in this type of situation is the one with more value between the presents.
He asks Jane to call him Tom too.

Oh, and Sophie says that RJ doesn't like adversities, but we know that RJ likes to play his mental games, so I assume that RJ only likes to play when he knows that he's going to win. Remember the reaction when he loses the rock, paper or scissors? Silly, I know, but its worthy to mention, it's the only time we saw him angry.
The interrogation scene in S01E02 where McAllister gives a look to the person and she starts talking is a bit strange too.

And that's it. I'm probably wrong, but that's just some notes I made that I'd like to share with you. Sorry for any mistakes, english is not my native language.

The fact that McAllister appeared so briefly as a seemingly insignificant one-shot character in the series' second episode, and is now suddenly an important figure at the forefront of the Red John investigation is a pretty solid indicator that he is likely Red John.

His strangely aggressive behavior with Grace Van Pelt outside the winery, during that second episode (Red Hair and Silver Tape) is never explained, and even though I've always found that scene suspicious, I was inclined to dismiss it as a plot device to give Lisbon a plausible reason to leave the motel room she and Jane are staking out. Jane is then forced to confront the husband and wife killing team alone, which makes for a better climax when Lisbon then kicks the door in and is forced to kill them both. I'm beginning to think that my initial gut reaction to that bizarre behavior McAllister exhibited was bang on, and I should have listened to my intuition from the get go. If Sherriff Thomas McAllister isn't Red John, they're definitely buddies.

Red John tried in season 4 to take PJ at his side.Bret Stiles says to PJ that it seems RJ is jealous of PJ and thats why he takes everything from his life(i think this was on season 2 or 3 dont remember really well)and thats a form of love.Lorelei also said to PJ that after all this years RJ doesnt sees PJ as an enemy.In the case of the San Joaquin killer PJ in a way asked a favorite from RJ and that favor was to kill the San Joaquin killer.And RJ did it.You will wondered what's my point from all these events.It seems that RJ has a really weird connection with PJ and doesnt have him as a stranger.Maybe its not something huge but when PJ enters at Bertram's office and give his hand to Sherrif McCallister and says "sherrif mccallister" he says call me tom.Again maybe its nothing but it was really clear to me that with this gesture by McCallister looks like he says in his mind "come on we ve been a lot of things all these years Patrick dont calll me sherrif mccallister just tom for you"something like that.I also thing that this Carmen Lee thing wasnt random at all.We will find in the episodes i hope!

Tyger! Tyger! burning bright 
In the forests of the night, 
What immortal hand or eye 
Could frame thy fearful symmetry?  William blake (The Tyger )
SHERIFF SAY to Jane

a lot of meta in my area of forests

Ok, First of all. Let us assume that all what Bruno Heller said was true. He said that one of the 7 suspects is infact Red John. Let us do an elimination process by connecting the things Heller said.

Brett Partridge - Heller said he is dead so let us cross him out of the list

Bob Kirkland - On 2011, Heller said that Red John is someone we know and Kirkland appeared on 2012 so let us cross him out as well.

Reede Smith - We all know that he cannot be Red John since he is too fat to fit the Red John we saw who kidnapped PJ.

Bret Stiles - Sophie Miller said on the recorder that he is on his mid-thirties and Stiles obviously is not.

*That leaves us with 3 Suspects: Bertram, Haffner and Sheriff Tom McAllister.

Most of you thinks it is Haffner which is possible because of the whistle he did after talking to Lisbon. And he calls her Teresa just like Red John does. And when Sophie Miller said on the recorder that he is a great whistler, they showed Thomas Mcallister so you all assume that it is Haffner since they didnt show him.

But they obviously meant to do that so that we will think it is Haffner. I mean we all know how the Mentalist writers think, and it is usually difficult to predict who is the murderer in each episode.So I dont think they will make it that obvious.

That leaves us with Bertram and McAllister. 
Personally, I think that Bertram is innocent just in my opinion. So I'm going with Thomas McAllister. But its still possible it is Bertram or Haffner since there is no valid point that they cant be Red John.