At the end of the Season 2 or 3 when Jane is knocked out and tided to a chair and RJ comes, he looks with a medium build not heavy (fact corroborated by Rosalind, the blind lady that had a physical relationship with RJ.  She has said that RJ has a slight build but athletic), so that excludes Gale Bertram and Reede Smith. Brett Partridge and Bob Kirkland are dead so that excludes them (Unless the guy who was shot in the last episode was not really Bob but his twin brother…) Bret Stiles is too old and Thomas McAllister was excluded since he was not acrophobic… If all this is right… Ray Haffner is RJ

There have been several clues in different episodes.  The one I keep going back to is the blind lady that had a physical relationship with RJ.  She has said that RJ has a slight build but athletic.  No way could that be Brett, he's got to be too old and wrinkly.  I think the Sheriff McAllister is too old also.  Lisbon and Jane were reading Sophie's notes and said that RJ was an accomplished "whistler".  When Ray Heffner went to visit Lisbon in the hospital after being "stunned" by RJ, he left her room whistling.  Also since RJ has quite the history going back several years, I don't think he would be too young.  If it's one of the people on Jane's REAL list then I vote Ray Haffner.

i love this show but this should be the last season of the Mentalist.

Jane should find RJ, kill him, and not get in trouble for it.

He should then bring down the Tiger organisation in the finale.

The last scene should be a skip forward a year and show Jane with Lisbon and a baby, he started the show with nothing but vengeance and it ends with him happy.

Who wouldn't want that ending ?

All those clues about Ray Haffner are good, I can agree with most of them that is why I won't repeat them, but the most striking one for me is :
RJ did not kill Lisbon, he doesn't do that and we know well that Haffner has feelings for Lisbon, that is why he left her alive.

I made some observations, some of which are old and some of which are new. They make me feel that Red John is Haffner.

1. If you see the scene where Rosalind calls Jane and says "Roy is here", RJ sits with tea cup in his left hand. Look at the way he sits. Now compare this with the way Haffner sits when he visits Lisbon in her office. He sits exactly the same way. The camera will show you that for just a few seconds, when he is talking we see only his upper body. I think this could be a vital clue

2. He is a surveillance expert. He can watch people using all his resources, that is why he is ahead of Jane.

3. He was 21 and was in Elliston Farm when he was a member of visualize. He tells Lisbon that he doesn't believe that Brett Stiles is God, but happens to believe in somethings he says. All other members of Visualize believe Brett Stiles to be God, only RJ can say like this.

4. The way he walks in the hospital in Season 6 Episode 2 is exactly the same way Red John walks in Season 2 Episode finale, where he kills RJ copycats.

5. Well built, not too soft or too strong, his hands look exactly like Roy, medium aged man, height is just below 6, short hair, good looking. All this fits him. 

I have always suspected him.  After the 2nd episode this season with Lisbon in the hospital and the 1 on 1 Jane had with Kirkland last nite that mentioned someone who was persuasive and quasi-religious I am 90% sure that it is him.

The only other plausible explanation i can come up with now is Kirkland's twin brother.  Also lost in that 1 on 1 was were Kirkland mentioned that his brother was sadistic and twisted.  Loralei could have seen Kirkland and Jane together and not realized that the Kirklands were twins and assumed it was Red John.

Ok, then lets start.

Brett Partridge----> DEAD (RJ associate)
Robert Kirkland----> DEAD actually hunting RJ on his own
Reede Smith----> ALIVE ACTIVE (RJ associate)
Sceriff Mcallister----> ALIVE ACTIVE (maybe RJ, phobia of birds)
Ray Haffner-----> ALIVE ACTIVE (maybe RJ, good whistler RJ, Visualize member, right age, was in the farm)
Bret Stiles----> ALIVE ACTIVE too old
Gale Bertram----> ALIVE ACTIVE too old, plus he is bald and RJ has hair

No I will discuss why in my opinion Ray Haffner is Red John and why the other are not.

Reede Smith is clearly an associate but not Red John himself. He is merely an associate.
Sceriff MCallister could be RJ because of his phobia (birds) which are shown to be at the house where Partridge got murdered, that's the reason MCallister couldn't kill Lisbon. (weak clue but keeps him in the game)
Gale Bertram is too old, plus he is bald and Rosalind Harker said that Red John has short straight hair. Not Bertram. Plus Bertram is an high rank CBI officer, is good to keep Red John informed about everything happens into the CBI and by this I mean everything regarding PJ and PJ cases. He is a member, of course he is. But he is not Red John.
Bret Stiles is too old to be Red John plus he, like Bertram is way too on sight. He has a public face. Can you imagine someone pupular going around and killing people? I think not. Bret Stiles created an organization (Visualize) that prepars people to become eventually Red John's disciples. He is an accomplice "I know more than you thing about you and Red John" said to Patrick in one episode. So he clearly is involved. But he is not Red John.
Ray Haffner is definetly the best option. Right age, he is muscular but not too much. He has got short straight hair like Rosalind said. He is smart and is a member of Visualize. He was in the farm in the same time Red John was and by not aswering Lisbon's when she asked if he was one of the boys at the farm... indirectly confirmed it. He worked for FBI, for CBI and now he is on his own. He placed his mens, he recruited them while he was in FBI (Smith, O'Glaughling) and in CBI (Bertram).
He is a good whistler. Rosalind said he had rough hands. He is shown lifting weights, why they show us that frame? He is Red John. 

Robert Kirland\Michael Kirkland I am amused reading others saying that Mike is Red John's right hand. Because is true and you all really smart figuring it out so quickly. Robert is sneaky but only because is on a hunt, and he knows is a deadly one. Michael being his twin is no different like him. When he asked "you know me" to the guy in the ospital was because if he aswered yes then I would have known that his brother was in the right direction but by saying yes he got assured that the RJ associate hadn't said anything. Michael is alive and active.