Well... I still have a few episodes to go before I finished watching Season 5 of the Mentalist. 
I know, the most obvious theory isn't always correct, but in this case... I have two suspects that I really like for this.

1. Bret Stiles
Lorelei said that Patrick and Red John were much alike, manipulative and playing mind games... When she escaped from prison and was in the Motel with Patrick Jane, she said a few things that totally reminded me of Stiles' cult: Visualize. She said after her sister was killed she "found her true self. Nothing can hurt me". That sounds much like Visualize. 
And Bret Stile surely knows how to play with words and people's minds. 

Although... I think he's somewhat too nice for that. I came to sort of like him in the end.



2. Bob Kirkland
Why? Well... He's high up, if I'm not mistaken. He could have the powerful friends that Red John has throughout Law Enforcement. He pushes the CBI off the Red John case, doesn't want anyone to talk to the only witness who could identify Red John, after Lorelei shot the man. (Sorry, I forgot his name). 
And he was the one at the hospital, asking these weird questions, if he had ever seen him before and if the man would remember him. Why would Kirkland kill the only living suspect who can identify Red John, if he was chasing that serial killer? I believe he either IS Red John himself, or he at least is a close associate of Red John's. 


These are just my main suspects, but I know the producers of the Mentalist do know how to keep their fans on their toes. And after all... Gale Bertram might be a possible suspect, too. After all, in the beginning of Season 4 (I think it was S 4) he quotes the second verse of William Blakes' "The Tyger" and states it's just an old poem he likes. So maybe he could be Red John too.... but he almost seems too nice for that....

The name Robert means "fame-bright."  It is interesting that the poem that I choose to recite to Jane is The Tyger, which begins, "Tyger! Tyger! burning bright / In the forests of the night, / What immortal hand or eye / Could frame thy fearful symmetry?"  The similarity of the content of this poem with the meaning of the name Robert is important, I think.  Kirkland comes from a Scottish origin meaning "church."  There is an obvious connection between Visualize and myself, I just wonder if anyone can see just how far those ties go.  Then again, I might just be trying to throw you off my trail.  It appears that many people have already correctly deduced who I am, but I don't want to spoil the fun just yet.  I just thought I might give you some additional things to think about -- clues, as it were.  Happy hunting!

☺

What I have in my head now is quite tangled but, however, here's what I've got:
- BRETT PATRIDGE may be the most viable suspect. According to Rosalind Harker's description, RJ is just under 6 feet tall and Brett is 5'11'' and also he is not muscular, but not soft either. About hair, well, Brett's heir is straight, but long, but he could've had short hair when Rosalind last talked to him. NOT MATCHING:  BP's voice is not the really soft voice from the description, that you would've expected. If he's a serial criminal, why on earth would've he chosen such an obvious job such a forensincs investigator, because it is useless if he never gets to his own crime scenes to check for the mistakes he could've done. He only was at the fake RJ crime scenes. POSSIBLE TRUTH: He is only a looser who is made to look too obvious to lead us into error.
- BRET STILES is also just under 6 feet tall, more exactly 5 feet 8 and a half inches and also he is not muscular, but not soft either. NOT MATCHING: His voice is not soft at all. He might've committed crimes, but not with his own hands. He is very kind around Jane and the agents. He's got no actual reason to commit all those crimes, I mean not this way. 
- BOB KIRKLAND also matches the physical traits and has a soft voice. Why didn't jason Lenon recognize BK as Red John? Because probably he is not RJ himself and is only working for the latter or perhaps he is indeed RJ, but he doesn't show his face to any of his adherents, or he's had plastic surgery, but very unlikely. What about the copies of Patrick Jane's investigation board papers? Maybe BK, in this case RJ, needed them to get one step ahead of Jane and predict the latter's 7 names list, or perhaps he only wanted to carry on his own investigation. NOT MATCHING: BK does not kill Jason Lennon in RJ's way and does not draw a face on the wall, but it may be because he has no blood for it. The most important thing which doesn't match is that PJ has met and shaken hands with him, but it was just before he's got the clue from Lorelei, who was unlikely to know that those had met.
- RAY HAFFNER is just over six feet, but he wants to get out of CBI, which is a sign the he(if he is RJ) is afraid of Jane's progress towards him. Also he has been at that farm where Jane has found RJ smiley face on a barn. It was mentioned that some kid made it. Ray was in the Visualize by then and he was around 21 or so,which makes him being taken as 'some kid'. NOT MATCHING: as I said he is just over six feet tall(6'1''), he is quite muscular as his job requires.
- REEDE SMITH's name is an anagram for "the red is me" or "theme is red", which suggests a connection with RJ. NOT MATCHING: He is too tall(6'3''), and RJ is not fat, he is between muscular and soft, as it can be seen in season 2 when the latter saves Jane.

- Gale Bertram and Thomas McAllister are either accomplices of RJ or just names to fill the list.

Just nowI have simple reason to say Kirkland is not Red John Lorelei said to Jane in season5 episode8
 I only wonder why the two of you didn't become life long friends the moment you shook hands
 in previous episode Jane met Kirkland for the first time after Lorelei release ...Lorelei in the prison ,,and by the next day she escaped and spent whole day with with jane ,,how she knows that jane met Kirkland unless he informed her in the prison ,,,may be he knows RJ or he's assistance but isn't He,,,,, .....
 Kirkland said sorry mr Jane ,..jane asked him
 do u I know u,,,Kirkland answered no but I know you ,,,,and secondly in next episode last scene Jane asked Lisbon about Kirkland who's that guy !!!!

We all know that Bob Kirkland works for Homeland Security. So he has to have access with the person who sees everybody in the cameras. He looks & probably make some of them targets.
That's why I think Bob Kirkland is Red John

When jane first met Bob, he looked at him suspiciously and asked him I f he knew him. Bob answered by saying that he knows him. And then they shook hands and there was kind of an emphasis on that handshake. The following episode , jane learns that he shook hands wiyh red john. Also in the 99th episode we can see bob kirkland talking to the head of the fbi telling her to talk to the head of the cbi to keep updated on janes work in the red john case

I believe Red John is Bob Kirkland just for the reason he invaded Jane's "office" and took pictures of all of his work. He was smart enough to copy exactly what Jane had in there. And if he could do that he also could have thought of the list that Jane has. Only Bob could have such information, same as Jane. Jane also knows his "office" was broke in and someone his information because of the bit of the toothpick he left on his door, let's see what happens from now on. And remember that Bob always has a suspicious behavior and always wants to be informed of news on Red John's case.