Brett PartridgeBrett Partridge : Since the airing of "Red Sky in the Morning, which marks the first time Red John is heard speaking as he recites "Tyger, tyger" to Jane, Partridge is suspected by many fans to be Red John because of their voice similarity (and similarity of RJ's face in a frame of His Red Right Hand). RJ's pronunciation of words and letters ("s" for example) is almost the same as Brett's. He appears in the two RJ copycat's scenes. Additionally, Jack Plotnick, who plays Partridge, is 5' 11" tall and ; in Red John's Footsteps, Red John was described by his old lover, Rosalind Harker, as being just under 6 feet tall. Brett appears in Jane's List of Suspects in Black Cherry (his name is the first visible). The surname recalls a picture "A brace of partridge" of RJ's favorite poet and painter, William Blake. Also, the scene with Jane in the beginning of episode 2x23, when Jane surprises Partridge by coming back suddenly from another room is acknowledged, by many, to be a mirror-scene with the scene with RJ in the end of the episode, when the killer seems to go away then comes back surprising and scaring Jane. Additionally, Brett Partridge makes his appearance only when there is a fake Red John at play, probably to look for clues and find the imitator. In relation to William Blake, Partridge seems to like Charles Dickens, another famous English author and poet. Could there be a connection?This is a very possible scenario. 
Everything points to him, or is that just what everyone wants us to think? Jane is smart; he would have figured it out, and I bet he analyzes specific things in everyone he meets to compare them to Red John, given his past. We can't know for sure if Jane suspects him or not. On to the physical aspect. Roselind Harker did describe Red John as Bret looks. Average height, short, straight hair, neither strong nor weak, just softish. The calloused hands would make perfect sense for Bret, given his job. He can be Red John, or an accomplice. Also, he's perfect for the descriptions as an accomplice; O' Laughlin was FBI, he couldn't really have gotten deep into the CBI. Bret could. Big possibility. In "His Brett expresses his admiration for Red John in the scene of his copycats. Right Hand Man", someone shows up to kill Rebecca. Obviously Red John or an accomplice. Rebecca could have reported who he was. On the surveillance feed, it shows a man just under six feet tall, of average weight and build. He is holding a newspaper, topped with a cap covering his face and wearing a windbreaker that possibly says "FORENSICS", just like Bret Partridge in "Red Sky in the Morning". Even the way the man walks resembles Bret, but then again, there isn't much difference on how people walk.Now, here is a video that compares Red John's voice and Bret Partridge's voice: http://www.youtube.com/watch?v=L6hU_l2wWwQNote Red John from "Strawberries in Cream" once more in a clip, and see how the voice he uses when talking to Lisbon is a bit off -pitch, maybe higher. Here is the URL to the clip:
http://www.youtube.com/watch?v=7rjXOkCxUUwWilliam Blake, A brace of partridge. Blake is RJ's favourite poet (he quotes "The Tyger") evidence of his similarity to red john is the voice, this childish character and behaviour (RJ killed only for a critic of his work), this psychopatic mind, this hate for Jane, the picture of Blake "A brace of partridge", his physical appearance. He's apparently not very intelligent and useless, but maybe he is hiding something... He's a supervisor of CSI (he said "my guy") or a responsible officer, anyhow. According to his job, I presume he also has a degree in biology or criminology.AND THIS IS THE MOST IMPORTANT PART:The great evidence, I think, almost proof (I don't understand that Jane doesn't see) is the scene of 2x23 with Patrick: it's a mirror scene of RJ and Patrick scene, when John speak of Kristina.
Scene of murder of Marley Sparrow. Brett and Jane.
Brett Partridge (sarcastic): Yeah, that's great, dude. But where's Red John?Patrick Jane: Oh, this wasn't Red John.Partridge (sarcastic): So you say.Jane (He walks fast to Brett and looks serious): So I say.(Jane walks away)Partridge: Jerk.Jane (returns, it seems that he forgot anyway, startling and slightly scaring Brett): Oh, make sure you have your people send those papers and journals to the CBI office.Partridge: Will do.Jane: Thank you.(Jane exits the room)  
Scene of copycats; Red John has killed his copycatsJane's reconstruction of Janet Peak's murder by Red John. His true face is similar to Brett.Added by JoeastorRed John (at Jane): Do you know who I am?Jane (soundlessly): Yes.(Red John uprights the chair, whichJane is tied to)Red John: Tyger! Tyger! burning bright / in the forests of the night, / What immortal hand or eye / Could frame thy fearful symmetry?(Red John exits, Jane is afraid)(Red John returns and startles Jane, putting his hand on Jane's shoulder)Jane (stratling and soundlessly): Ah!Red John: I almost forgot. Kristina, would want me to send her love.She's very fond of you, despite your difference. Roll Tide! (gives two friendly blows on Jane's shoulder. He exits and Jane relaxes his breath, smiling because he's not dead).  The writer has said that when RJ appear, has get some red herring, that the audience will be disappointed, because RJ is only a simple man, that "simplicity is sophistication".
Edgar Allan Poe wrote that sometimes the best way to hide something is to put it in plain sight. (The Purloined Letter). Red John is a showman, says Patrick. Heller was inspired by english actor and comedian Ricky Gervais to create Partridge, the one of the few minor characters who has a name and surname, and appears in the seminal Pilot. Brett remember the Joker. It's a psychopatic man. Brett is suspect.

Writers have compared RJK to Waldo, a famous character for children who appears in the background. "You've got to look at it like Waldo" as Ashley Gable once tweeted in the offical Mentalist Writer twitter account.

That being said, characters like Brett Stiles or Walter Mashburn couldn't possibly be Red John. They're not background characters, they're simply "out there" and are target of suspicion to lots of people. It's almost too obvious. Same thing goes for other characters who have a strong presence in the show, such as Bertram or Minelli. Personally, I would rule out anyone who is highly suspected by a large number of people.

On an unrelated note, I also think it's unlikely that Red John is someone we've only seen once in the show, such as Dr. Royston Daniel, simply because it would be too disappointing. 

Red John can't someone of too strong presence in the show, but neither a random guy we've seen in one single case.

I believe that Red John is Robert Kirkland. Why? Well first of all he matches Rosalind Harkers description, he also is very well connected in Homeland Security and can go places without raising suspicion. He also killed Jason Lennon and asked him several times if Jason recognized him. This leads me to suspect that Kirkland is not his real name and that it is just one of his fake identities. He probably used another follower to act like Red John with Jason and just wanted confirmation that Jason did not know the "real" Red John. He also is not listed on the season finale cast list, which leads me to suspect that he will appear but that it is supposed to be a secret just like with the fake Red John in the season 3 finale.