Hello all. I'm not gonna theorize on who is Red John, but i rather want to try to eliminate some of the suspects.

1. Gale Bertram - he used to be
my number 1 suspect before season 6 aired. After discovering he is
member of Tyger, tyger collective and connecting that with some
previous events, I now start to believe he's either a just a low level member of TT or maybe a good guy actually, on undercover mission there, but not Red John. Here's why:

  
                       - in S06E02 he's in office with sheriff and
Reede Smith discussing something and then Patrick bumps in saying he
made progress on Red John case. Gale replies: ''Maybe...You and I should
discuss this alone.'' Note that all of them (except Patrick of course)
are members of TT, then why would Bertram wanted to discuss Red John
matter alone with Patrick? If he's a true TT member, he shouldn't have
had any problems discussing it among sheriff and Reede, since they are
all TT. Obviously he didn't want them to know Patrick's progress
on Red John case.
                          - in same episode, in
same scene, but after Patrick left the office, Smith says: ''Exactly how
much does Jane know?''
Bertram replies: ''Well, that is the
question.'' and to that Smith reacts with anger (like he is superior to
him): ''It is your job to find out.'' I got feeling here that Gale is
just a relatively new member of TT, lower level than Smith, with a task
to use his CBI position to watch on Jane and to get as more information
as possible about his Red John investigations and to inform TT about it.
                       
  - in S06E01 when he came to Lisbon and Jane, he said to them: ''I
know that sometimes I must appear calculating, even duplicitous to you,
but please understand it - it comes with the job description.''  With all of the questioning Jane is he close finding Red John and how he wants to be there when they catch him, he understands that they probably think of him as a possible suspect, but he doesn't want to jeopardize his position in TT (the case if he is actually on undercover mission there)
                          - in S05E16 we see Bertram questioning Lisbon about their Red John & Lorelei progress, do they have any leads while Kirkland is hiding. We know now Kirkland was ''good guy'', was hunting RJ himself as well and Bertram helped him and let him use CBI resources to catch Lorelei. And after Kirkland said about Lisbon: ''Can we trust her?'', Bertram replied: ''Can we trust anyone?''
While both aware of TT back then, knowing how huge this organization is they didn't really know who else within law enforcement is member of it, that's why Bertram said ''Can we trust anyone?'' They were suspicious about Lisbon, and that's why Kirkland was hiding. If Lisbon was TT and saw Bertram(also TT) with Kirkland, Bertram would have probably ended up being dead (same as later Kirkland) because of betraying TT. Note that some of the members of TT don't know each other if they don't use the phrase. 
                         -  back in S05E1 Lorelei is captured and there's ongoing battle between CBI and FBI for her custody. Reede Smith (FBI, TT) desperately wanted her in FBI custody, away from Patrick. But, if Bertram was true TT, wouldn't he want her away from Jane as well? No, because he actually fixed him having one-on-one with Jane.

If Red John is within Tyger, Tyger and he is their leader, then Bertram can be safely discarded as possible Red John (I don't think he even knows who is RJ). But, if Red John and TT are 2 different entities, then Bertram could be Red John (part of TT without them even knowing Red John is amongst them). However, i believe RJ is TT and their leader and that's why i eliminate Bertram.

2. Reede Smith - he is probably high up in TT (remember how he gave orders to another TT member - Bertram), and probably a close friend of RJ. Remember The Crimson Hat and Red John: ''I have a good friend in FBI, so i found out when FBI found out.'' Red John was probably referring to Reede Smith here, his friend in FBI.

Ok, now what i think of others.
 i don't think it's gonna be Stiles (he's already dying from a disease),
 nor do i think it's Haffner (him and Kirkland made the most obvious RJ scenes ever - back in Red Barn where we learned RJ was in visualize then boom, at the end of very same episode we learn Haffner is also in visualize and he didn't want to answer if he was at that farm, i mean c'mon... and Kirkland with his handshake - those 2 are the most obvious choices and The Mentalist was never that easy, never that obvious).
I give the highest chance to sheriff - I always believe rj has to be some1 from first 3 seasons, and he first apeared in 2nd episode! Mysterious, calm and almost forgotten. In his all appearances he doesn't give out anything to believe he is Red John, he's very careful, unlike other suspects. This show is aiming to shock us, and who of all can afford to shock you but sheriff? No one, others are too obvious.

And for end, i give 1 % possibility to Partridge.
 Patrick: ''If he's Red John, then he is a very good actor.''
Red John is master of crime scene, has never left a single clue, and Partridge was CSI lead agent. Always in wrong, in his every appearance, or just pretending to be wrong to not draw the attention?

10x for reading.