I think it is Brett Partridge now.  Here are my reasons why:
First of all, he is intelligent and so is Red John.  Why?  Because he is involved with forensics.  Forensics jobs take a lot of skill and intellect.  People with forensics probably have the knowledge to fake death.  In other words, Brett Partridge could fake his death.  He might have had an accomplice to help him look like Red John was murdering Brett Partridge.
Second, Brett Partridge voice is very similar to Red John's voice.  Don't believe me?  Watch  episodes season 2 episode 23 and season 6 episode 1 to find out.  In addition, Brett Partridge was in these episodes.  Coincidence?  I think not.
Third, Red John is probably in his 40's  and Brett Partridge is probably in his 40's too.  In fact, the actor who plays Brett Partridge is in his mid 40's.  In other words, Brett Partridge and Red John are middle age.
Fourth, Red John has been in the beginning of the Mentalist.  So has Brett Partridge too.  
Fifth, Red John has straight hair and so does Brett Partridge.  In addition, the body type and height are probably a match.  Red John is not fat but not too skinny either and he is about average strength.  Brett Partridge probably matches that description.
Sixth, we have not seen a body of Brett Partridge or at least a funeral for  Brett Partridge yet.
Seventh, Red John is a creepy person and Patrick Jane thinks Brett Partridge is creepy.
Final reason, remember the scene with the pigeons.  The CSI does have pigeons in his house.  In season 6 episode 2, there was a woman feeding pigeons and in season 6 episode 3, a pigeon did scare Sheriff McAllister.  What if Red John's favorite bird is the pigeon?


Now let me give you reasons why the other suspects are not Red John:
First, Bob Kirkland died.  He was killed by Reede Smith.
Second, speaking of Reede Smith, he is too fat to be Red John.  Also, he is probably an accomplice of Red John.  Do you know who else could be an accomplice?  Gale Bertram and Sheriff McAllister.  In addition, Gale Bertram's hair style does not match the description of Red John because he is bald.  Also, Sheriff Mcallister and Gale Bertram are a bit too old for Red John.  Rosalind Harker was a lover of Red John and she is early middle age.  Do you think these two men would love Rosalind Harker?  I do not think so.  Furthermore, Sheriff Mcallister is not afraid of heights.  Red John is afraid of heights.
Third, do you know who else is too old for Red John?  Brett Stiles.  However, he could know who Red John is.  In addition, he maybe the father of Red John.
Fourth and final reason:  I originally thought it was Ray Haffner until I found out that he does not have the tattoo.  However, I am not ready to rule out Ray Haffner yet.  Why?  Because he is a good whistler and he does like Teresa Lisbon and so does Red John.  Otherwise, Red John would have killed her.  Another thing, Ray Haffner did appear in Teresa Lisbon's dream and he was the one who hit her.  In addition, he is middle age and is average strength.  Furthermore, he does have straight hair and is a member of Visualize and so is Red John.  Also, what if it was an accomplice that did Red John's dirty work at the end of the episode "The Red Tattoo"?  Remember, Red John is pretty intelligent and he can fool Jane and the audience who watch the mentalist.

Overall, I am thinking it is Brett Partridge but I am not ready to rule out Ray Haffner.  If Ray Haffner does not survive the explosion, then I rule out Ray Haffner.  If Brett Partridge appears again, it confirms my theory.  Anyway, please comment and critique on this theory.  I would appreciate it.