As many people have already done I would like to put in my two cents about each of the final seven suspects.  This is mainly because Bruno Heller has stated categorically that Patrick Jane is NOT Red John (something I have waited for, for a long time) and because he stated that one of the seven final suspects is indeed Red John {see: http://insidetv.ew.com/2013/05/06/mentalist-finale-season-6-red-john-list/} (putting paid to my [and others] immediate theory that Jane's suspect list was a fake to throw off Red John)

Bret Stiles

Head of Visualize.  Previous inside knowledge of Red John.  Likely killed, or arranged the killing of, the former leader of Visualize and the Sheriff who investigated his death.  Helped Jane break Lorelei Martins out of prison.

I think that Stiles is a strong candidate due to his position at Visualize.  This fits in with the concept of hypnosis or entrancement that has seemed to be a feature surrounding Red John's accomplices (in that they are all fanatically devoted to him and many are willing to die to protect him/his identity [see Strawberries and Cream when the assassin committs suicide])

Gale Bertram

Bertram was my main pick for being Red John's accomplice in the Season 3 finale and I was immensely disappointed when it turned out to be O'Loughlin (except for the fact that he died as the Craig/Grace storyline was extremely annoying to me)

Director of CBI.  Previously a Red John Mole suspect. Talked about how he "gutted" someone at poker (as a professional poker player myself this is not standard poker terminology) and seems consistent with Red John's MO.

Another reason for my suspecting Bertram was the possibility that Red John knew that the Pacific Palms Hotel sting operation in Strawberries and Cream was a trap.  Timothy Carter (Posing as Red John) said to Jane "when O'Loughlin told me of your little TRAP I was so pleased.  One more chance to teach you a final lesson in humility"  To me it is significant that Carter said "trap" instead of something like plan, or scheme.  It suggests inside knowledge to me as the only other explanation I can think of is lazy script writing.  Therefore I thought Bertram was a good candidate from (from Season 4 onwards) to be Red John or another of his accomplices.

Overall I think Bertram is another good possibility as his position as Head of the CBI (and likely other high ranking law-enforcement positions) underline a theme about Red John that has been present since almost the beginning of the series.  Also, something I didn't mention above was that he quoted a line from "The Cradle Song" by William Blake who Red John himself quoted in the Season 2 finale.

Raymond Haffner

Supervising CBI Agent.  Left/leaving for Private Sector (asked Lisbon to join him and mentioned it was for much more money)  Member of Visualize. Possible connection to 1988 Red John killings at Elliston Farm.

Despite having only two episode appearances Haffner also seems to be another strong candidate (though not one I want to be correct).  As a member of Visualize and an important person in the CBI (and probably the wider community also) he would likely have a good personal relationship with Bret Stiles which could explain where Stiles is getting his inside information on Red John.

His possible connection to Elliston Farm fits around his approximate age and the age at which he said he joined Visualize (around 17 or so if I remember correctly).  Also the fact that he asked Lisbon to join him in the Private Security business is consistent with Red John's attempts to destroy everything Patrick loves and holds dear.

Overall ANOTHER strong candidate (this detective malarkey is pretty hard!) that seems to fit many of the criteria that have become common knowledge to most regular viewers.

Reede Smith

FBI Agent, partner of Gabe Mancini.  Only appeared in the Season 5 premiere which was shortly after the failed Las Vegas sting operation.  Apart from the law enforcement connection, the timing of Smith's appearance on the show is the only real [intangible] evidence pointing to it being him.  It could be that Red John was trying to observe Patrick directly (something he clearly enjoys doing based on previous actions) and find out if the events in Las Vegas had left him any closer to finding him (Red John) also, was I the only one who thought Smith looked faintly amused by the whole "Red Bead Incident" in 5x01, which would make a certain amount of sense if he was in fact Red John.

Overall, not much to go on, a lot of conjecture and no real evidence or connections.  I hope he is merely a red herring as having only two/three suspects to look into would be perhaps too easy!

Bob Kirkland

Agent with the Department of Homeland Security.  He has engineered it so that he has been kept apprised of the Red John case for over five years (see "Red Dawn" Limo Scene at the end of the episode)  Killed Jason Lennon (Red John accomplice who likely had key information about Red John)  Had Homeland Security Agents break into Jane's loft and copy his Red John "project board" and made his own version of it.

Kirkland is the only person on this list I personally believe is definitely NOT Red John.  This is because before he killed Jason Lennon he asked him [Lennon] if he knew who he [Kirkland] was ad Lennon said that he didn't.  Lennon likely knew at the very least what Red John looks like as he delivered Lorelei's sister to him.

I believe that Kirkland is either a high-level Red John accomplice/protege/partner or that he is himself hunting Red John in the same that Patrick is.

Sheriff Thomas McAllister

Only appears in 1x02 "Red Hair and Silver Tape".  He seemed interested in Jane's "psychic" powers and lost at Paper/Rock/Scissors every time when he played against Jane.

McAllister has the law enforcement connection that seems to be highly relevant but as he only appeared in one episode I believe he is likely to be another red herring.

Brett Partridge

Last but not least....

Partridge has been a suspect of many fan theories since most probably the first episode and as a results much has already been said.

He is the epitome of the "Creepy Forensics Guy" persona.  He has worked Red John cases (copycat  and genuine) He quotes classic literature like Moby Dick and Charles Dickens like Red John with William Blake.  He appears to have a strong dislike of Patrick Jane (although this seems to have mollified in 5x22) and he also appears to revere Red John as seen in 1x01 and the Season 2 finale.

He has some voice similarities to Red John's voice in the Season 2 finale.

The William Blake painting "A Brace of Partridge" could have been the inspiration for Red John to take the name "Brett Partridge as an alias.

Overall I think that Partridge is another strong contender, he seems to fit the thoroughly macabre nature of Red John (noted by Patrick Jane in his first list of suspects) and his [likely high-level] position in the CBI forensics team would give him huge access to the CBI archives, computer databases and evidence rooms.  The William Blake and other literature connections (not just through Partridge) throughout the series seem to me to be too frequent to be random and are there for a reason (whether as red herrings or otherwise) and he seems to fit Rosalind Harker's description of Roy Tagliaferro perfectly.

Well, that is my two cents on each of the final seven suspects and as Bruno Heller has confirmed that Red John is indeed one of them we are certainly in for a roller-coaster of a season when September comes around!

Please comment and let me know your thoughts and whether I have got any information wrong and link me to your theories as well please!

Happy Hunting!

Pyrites.