According to me, Bret Stiles is Red John. 

Following are the reasons for the same:

1. Remember Bruno Heller said : You would be disappointed to know the identity of Red John.
(Everyone is rejecting Stiles coz he's too old)

2. Lorelei Martins told Patrick Jane in Season 5 Episode 8(Red Sails in the Sunset) : "GOD, you're just like him, you know that".
(There's no one else in the list who is like Patrick Jane except Bret Stiles)

3. Bret Stiles was accused of murdering the old leader of Visualize.
(He has the motto and he killed every witness and destroyed every proof in that case)

In the latest episode Fire and Brimstone, Sheriff McAllister, Bertram and Smith have tattoos. So that means they are working for Tyger Tyger Organization and Smith has already confirmed of his side before killing Bob Kirkland. Now two suspects remains - Bret Stiles and Ray Haffner.

I don't think Ray Haffner is Red John coz he does not get fit into the character of Red John. He's acting a little crap in last few episodes.

The very first murder by Red John was done in the late 80's in the barn where there was his smiley face. It is definitely done by Stiles and now he knows Patrick is very close to him that's why he's planning something else by dying from our material world.

My money is on Bret Stiles.

Please comment if you want to say on my theory.

Thanks.

I'd be curious to see more discussion on the comments from the poster who wrote the "spoiler" based on cast list (quoted below). I have a hard time accepting Bertram as RJ. Seems like he was an afterthought cast addition once the show was underway. I also find it unlikely the more comments I read that any of the 3-dot crew are RJ. Heller was already frank about the list of 7 being real, so it's almost like he gets to throw us off with a "fake" list of three.....Considering that and based on other commenters, I am now leaning to Ray or Stiles with a small window of plausibility that it could be Partridge or Kirkland undead. I also like some of the theories that someone on Janes team is duplicitous, amybe not with RJ but maybe with Tyger... if so, I'd vote for Cho. We are all looking to be shocked about who is Red John that the big surprise could be something else.

"...spoilers for the next episode based on pictures, cast list, and sources.
dont read this if you dont want to know who the real rj is.
and i dont care about green or red votes because this aint a theory.
s607 - sherrif thomas mcallister dies.
s608 - reede smith gets arrested, red john will be gale bertram."

the whole RJ finale seems like a big disappointment. Someone that apperared in very few episodes the past 6 years, someone with no emotional connection with the main cast. RJ will prove to be either someone totally indifferent to the series (like Mcallister or Reede) or obviously suspect (like Bertram (he even said it himself he is duplicitous)) or Haffner-Kirkland (with obviously suspect behaviours in their few appearances) or Stiles (what a surprise, wow) or Partridge who was the main suspect from s01e01. I mean whoever from them is RJ, in my opinion the writing is just bad. No drama, no surprise, nothing! RJ is just another crazy bad guy. 

That said, there are other good series out there, fortunately!
P.S. I will take everything back if the writers surprise us with someone else, but I sincerely doubt it.

I think what people have been saying about the 666 theory is correct.
You see the numbers at the end of 6x01 where BH said there was a clue to RJ's identity. 

If you type Red John you get 666 and if you type Gale Bertram you get 666
If you type any of the other suspects you don't get 666.
http://www.openjs.com/scripts/666.php

Not taking credit just saying it makes sense.