I just watched the Red Barn episode, and now I think it's Ray Haffner.

1. He fits the physical description Rosalind gave, if we assume it to be correct.
2. He is on Jane's list of suspects, if we assume that to be real, complete and correct.
3. He fits the age group, as the "kid" who painted the smiley in the barn.
4. He's a member of Visualize and clearly RJ has some associations with the group- for example, Brett Stiles knew where Kristina Frye was.
5. He offers Lisbon a job and tries to make her detached from Jane, tries to make her think he'd leave. 
6. The most popular suspect is Brett Partridge, and he just seems too obvious, and too "goofy" to be RJ.

Let me share some possible clues I've found.

Season 1
1) In S1E1 Partridge believed the crime was made by RJ, which was a mistake. This reduces his chances to be RJ.
2) In S1E11 "He is Mar..." was on the hotel wall. There is no sense to write a surname (like Mars or Martins), because there would be no "He is" before it. So "Mar..." is something else. The most probable guess is "Marshall", which may refer to almost anybody from the final 7 list, except Stiles,

Season 2
3) In S2E8 an officer poisoned Rebecca (Bosco's secretary). Her smiling reaction directs to the conclusion that the officer was RJ. This speaks only against Stiles to be RJ.
4) In S2E23 RJ reads Blake's poetry to PJ. I see no reason to do that. The only thing might be to make PJ suspect somebody, who likes Blake (but as this point we hadn't met Bertram yet). By the way, RJ may be creating suspects by giving wrong clues ("He is Mar...", "Tyger...", else).

Season 3
5) In S3E3 O'Laughlin, while working on visualists' case, lets a guy shoot the member who was going to provide evidence against Stiles. So the man who was working for RJ worked for Stiles.
6) In S3E3 Stiles passed to PJ information about Frye. Can't make any conclusion from this yet, but obviously it's an important moment.
7) In S3E9 Todd Johnson's last words were "Tyger, tyger". After being burnt by O'Laughlin (and so by RJ), Todd would like to say something very important to PJ. So either Todd wanted to say that it wasn't RJ in S2E23 saving PJ, but Todd himself, or Tyger is a clue leading to RJ's identity. The only real thing is the letters "TYGER" or "TIGER" (this variation is also met in literature) being in the name and/or surname of RJ, for example "parTRIdGE".
8) In S3E16 Bertram directed LaRoche to suspect Hightower, which was a favorable scenario for RJ.
9) In S3E16 Bertram quoted Blake's poetry.
10) In S3E23 Gupta said he was very religious. This is a Stiles direction.
11) In S3E24 Timothy Carter (fake RJ sent by RJ) was at the mall cafe. So who did know about the meeting? Bertram knew. Besides, RJ knew about the "hotel floors trick" (because RJ sent Carter). So in the list O'Laughlin should not be the only RJ man. And again, Bertram is the primary candidate.

Season 4
12) In S4E1 Bertram asked the attorney to enlong Timothy Carter's case and probably was sure Carter wasn't RJ. This brings some increasement for Bertram's chances to be RJ.
13) In S4E13 RJ is shown to be left-handed or at least "equal-handed". Watching different scenes brings to the conclusions, that Bertram, Haffner, Smith and Partridge are right-handed, while Kirkland may be left- or equal-handed with a high probability. No valuable scenes found with Stiles or McAllister.

Season 5
14) In S5E1 Bertram tried to save Lorelei at CBI and not to pass to feds. This may be interpreted both as a RJ behavior and not RJ behavior. Anyway this may be a clue.
15) In S5E5 Alexa Schulz phoned Minnelli by Kirkland's request. The first idea is Kirkland to be RJ, but I would prefer another theory: Kirkland was trying to find RJ himself or to prevent PJ from engaging in RJ case and so saving him (see the clues below for more prove of this theory).
16) In S5E6 the transporter-cop told that a fed man called him and organized Lorelei's transfer. This was not to kill her and not to free her (none of these two happened). So the goal was either to interrogate Lorelei (this is a Kirkland behavior) or to put her far from PJ (also a Kirkland behavior).
17) In S5E7 Kirkland told Lisbon to stay far from Volker. Not an RJ thing to do, right? So Kirkland wanted to save Lisbon. What a kind guy!
18) In S5E13, which was about the case of visualists' farm bodies, it's mentioned that Martin Talbot said that a kid painted the smiley on wall. So nowadays the "kid" must be 30-50 years old. Not Stiles.
19) In S5E18 it's revealed that Kirkland knows RJ's identity (no question was asked about RJ before making the deadly injection).
20) In S5E19 Kirkland stole the results of PJ's investigation. But those results were probably fake, as PJ predicted the robbery.


So, let me conclude one by one.

Bret Stiles - Excluded due to his age. An alternative is that Talbot lied about the "kid" to prevent further investigations.

Gale Bertram - A lot of evidence talks for this candidate, but there is no sense in the manipulation with Blake's poetry. Mentioned above clue #14 is also a probable reason to exclude Bertram.

Raymond Haffner - See no reason to exclude him (except the clue #13, which may be not a hard one actually). To be honest, I can't see him being RJ because of S5E13, in which Haffner tried to get information from Lisbon about visualists' case by HIMSELF. Not an RJ style action.

Reede Smith - The same: see no reason to exclude except the clue #13. Actually the main reason to suspect him is his name. Which is the most famous fake name? John Smith (Red John, Reede Smith).

Bob Kirkland - Excluded. A very interesting character, but obviously not RJ. The first reason is clue #16. The second and more valuable is the fact that the disc record with Lorelei which we see in the last episode is made BEFORE Kirkland broke into PJ's room (combined with clue #19, Kirkland just wanted to find out how far PJ succeeded in his investigation). So, I Kirkland might be an ex-victim of RJ or an ex-companion. Anyway, Kirkland wants revenge. The "do you recognize me" question leads to the plastic surgery theory. So Kirkland may be Carter Peak somehow survived. Or Kirkland may be himself in the flashback scene, but later replaced by Todd Johnson. Martin Talbot may also be connected to Kirkland. A little fantastic, but it never seemed that Kirkland is a bad guy.

Tom McAllister - If taking under consideration only fact and forgetting that we are talking about a serial which should have its drama and special logic, I would say McAllister is the most probable suspect. There is no single reason to say he is not RJ.

Brett Partridge - If the series were to finish in season 1 or 2, then Partridge should be RJ. I think he was something like a plan B in case the project isn't prolonged for a new season. But after 5 seasons he's not going to be RJ. Not many facts to prove it, but the very first clue is reasonable.

So, the least possible suspect is Kirkland (only 2%), then comes Stiles (about 6%), nexts are Bertram and Partridge (12% each). Top three opened by Haffner (18%) and Reede (20%). The main suspect unexpectedly is McAllister (30%). He was nobody before the last episode, but now we should expect all the seven to be key characters in season 6, so it's actually not reasoned to say "he's not common enough to be RJ".
Everything said above is based on the belief that PJ's list is true.

Just something to toss into the mix.
Reede Smith anagrams:
"Theme is red"
"The red is me"

While he seems to have too quick of a temper to be Red John, he was conspicuously absent when Loralei was removed from CBI.

I write this post in the hope that I might be able to shed some light on many of the great Mentalist mysteries that prevent us die-hard fans from experiencing a proper night's sleep. I'm just going to jump right in.

First of all, I will address the ever popular list of 7 suspects. Bruno Heller (creator of the show) is quoted saying these exact words when asked about the list at the end of the season 5 finale..."Yes, we can trust that list. As much as much as Red John has been one
step ahead, Lorelei Martins’ intervention changed the game. So as much
as Red John is trying to play from a position of strength in reality
Jane has gotten a lot closer." ...well snap. There it is. I don't know about you but I'm going to trust that list. Then, I started thinking. Of course we can trust that list, even if Bruno was never quoted saying those words. Think about it. If Red John gave Patrick a DVD with a list of 7 names and his true identity wasn't on the list, wouldn't it be obvious who Red John is? If PJ had heavily suspected somebody and RJ didn't put that name on the list, then he just gave himself up! The ONLY way to truly disguise himself was to name himself among the 7. Like Bruno said, RJ is attempting to be clever and play from a position of strength but in reality PJ has him narrowed to 7 names.

Now I am obligated to discuss the "funny business." The "wait, maybe Bruno Heller is tricking us" areas in the show. I have heard so many theories about Lorelei still being alive, about Lisbon being one of RJ's accomplices, about Patrick making the video of Lorelei and the 7 suspects himself. I feel very confident that the list is very very real, made by RJ himself, that Lorelei is very very dead and has been dead, and that Lisbon is most certainly not one of RJ's accomplices or disciples. My reasoning: The video was most assuredly made by RJ because it was given to Patrick by that one lady (RJ's accomplice that stole the baby in the season 5 finale). PJ would have absolutely no reason whatsoever to make the video himself. I mean, after showing Lisbon he crushes the disk. Nobody sees it other than her. And it CERTAINLY wasn't made by Patrick to show Lisbon (like some theories suggest), because Lisbon is not an RJ accomplice. NO NO NO. That would kinda ruin the show because a major storyline in the show is Lisbon and Jane slowly falling for one another. So the vid was definitely made by RJ and Lorelei died when the show says she died. No funny business there.

Here is where it gets interesting. The show is predicated on "clever tricks" and "explainable solutions," not magic and mind reading and that sort of thing. So there will for sure be a logical explanation to how RJ did what he did. Here is my theory: Obviously, Red John knows that PJ is a smart guy. When he found out that Lorelei had told PJ about the hand shake, he knew PJ was gonna make a list. He knows that PJ is clever enough to narrow that list down to people that could ONLY be Red John or a RJ "disciple." I think that the list of seven names includes all of the RJ disciples that PJ has met and RJ himself. So 6 disciples, 1 Red John. This would be really easy for RJ to work out. He simply asks his disciples "Have you ever met (shaken hands with) Patrick Jane?" If they say yes they go on the list, if they say no then they don't. He got 6 disciples that said yes so there is your list, the seventh is RJ himself. He knows that PJ is smart enough to narrow that list down to only possible Red Johns, an lets face it, if you can possibly be Red John then you can possibly be a disciple of his. But PJ doesn't even know that that is what he has done. Am I making sense?? This theory would make perfect sense because every name on that list is sorta suspicious in some way or another. I'll go thru the evidence.

Evidence:
Ray Haffner; He had the whole Visualize weird moment with Lisbon. He doesn't seem smart enough to be Red John, but we got a look into his past and his membership at Visualize so I'm thinking disciple??

Gale Bertram; We know he has quoted some of William Blake's work in previous episodes. PJ also said that Bertram gives him "weird vibes" and when he first met Patrick he referred to him as our "boy wonder". Also, obviously his relationship with Kirkland is freaking suspicious. He doesn't seem smart enough to be RJ however. So, I'm thinking disciple??

Bret Stiles; Never know with this guy. Being the head of Visualize is certainly 1 strike against him and he's had numerous suspicious moments throughout the show. Always seems to know something about Red John and what he's planning. Smart enough to be Red John but I just donno. Definitely a possible disciple.

Bob Kirkland; Such a shady character. Holy moly. I don't like that he has come into the show so late and seems so Red John ish but you can never be sure. Everything he has done points you to RJ but I donno if he is in fact Red John. I think he was a big part of the "happy memory" somehow but I'm not even a little bit sure about that. Definitely could be a disciple, definitely.

McAllister and Smith; don't have too much to say about these dudes. We know that Patrick has met them so that is probably why they're on the list. Because they are RJ disciples that PJ has met. Simple. 

Brett Partridge; For some reason, this is my pick for Red John. I just can't quite explain it but every time he shows up all i can think is Red John. What I just can't stop thinking about is how Brett's first words on the show were "Red John enters here." Sent shivers down my spine. Also, in the season 2 finale, Partridge and Jane meet up early in the episode at a crime scene and Jane "scares" Brett (hard to explain I hope you know what part I'm referring to). He leaves the room and then walks back in and scares him. Later in the episode, Red John saves Patrick and then after reciting the Tyger Tyger poem scares him. Hmmm...interesting. I want to hear your guys' thoughts!!! :) Sorry for the lengthy post lol.