I've only read about 15 theories, so I don't know if someone else has the same opinion, but here's one of my guesses:
I believe the voice plays an important part in RJ's identity. Clearly he has an american accent (see the part with "Tiger, tiger, burning bright [...]"), so I guess Bret Stiles can easily be ruled out. As well as, maybe, the sheriff. RJ's voice is natural, there's no sign of (major) voice alteration on purpose. So, presuming that he indeed is one of the six remaining suspects, based on voice recognition I'd say he's either Bob Kirkland or Ray Haffner. Of course, there are obviously several other aspects to be analyzed...

Only if Bruno Heller didn't lie about the clues he gives in interviews it must be Kirkland:
#1. Red John is someone on the list of 7. Bruno Heller said we can trust the list.
#2. Red John has been there from the very beginning. He said we will realize that he has been there from the beginning. Partridge was in season 1 episode 1, and Kirkland was in the 'prequel' episode where Jane starts at the CBI.
#3. Partridge is dead, Bruno Heller confirmed the death after the last episode, that eliminates him from the above.
#4. Red John is someone we've seen before. We've seen Kirkland many times obviously.

The only thing that's baffling is that he also said that he knew who Red John is from the start, and even though Kirkland was there according to the story, he was not in season one, and that season was originally written as a stand alone story.

Red John is Bob Kirkland. From the previous episode the descriptions of Red is middle age in good health, doesnt have many friends, well spoken, excellent whistler, damaged narcisstic personality disorder, deceptive in emotional makeup, not needed for distractions. Clearly Bret Stiles is older than middle aged, even though he is a narcisstic individual. Bob kirkland is obviously middle aged, speaks well in a haughty body language, and very careful when he speaks. Its almost he had suffered from a speech impediment as a young child. Who agrees with me? :)

Does anybody remember the discription that red john had black hair? The episode that red john had revisited the blind lady it showed the back of red johns head. His hair was black!

The only suspect left with black hair is Kirkland.

Here are
some facts which, when putt together, might mean something and be a little help
in the investigation of Red John.



An anagram
for one of the meanings of the name Roy Tagliferro is "ruler
incognito," and that is how Red John seems to want to be like and he gets
other people to play by his rules. The last episode of season five is titled
"Red John's rules".

Roy means
red, therefore can Red also stand for Roy. One meaning of Roy is
"king" and of John "blessed by God ", thus Red John means
"the king blessed by God". The poem The Lamb by William Blake is related to the poem Tyger, Tyger, Burning Bright, as yin and
yang. At the end of the poem The Lamb,
the lamb gets blessed. Red John seems to see himself as the lamb and Patrick as
the tiger (the poem Tyger, Tyger was aimed
at Patrick). The lamb has a "tender voice" and he is "meek ​​and
mild". Anyone who knows Red John seems to perceive him as a gentle person,
while Patrick upsets almost everyone he comes in contact with. Red John might
want to make it appear as he is the lamb chased by the tiger.



Bret
Stiles, the leader of a cult, sees himself as a god. The symbol "the seeing
eye" is actually a symbol for the devil. An anagram of Bret Stiles is
"blest rites." Red John was a member of Visualize Self-Realization
Center, and Stiles may have personally "blessed" Red John. In one
episode, Bret says to Patrick that he knows more than Patrick and Red John
could possibly ever imagine. This statement makes two things clear: (1) Bret
Stiles sees himself as a god, above everyone else, and (2) he knows Red John
personally.



An anagram
of the name Kirkland is "dark link" and another is "dark kiln."
In the poem Tyger, Tyger was mentioned
something about brain (probably thoughts) created in an oven. Robert Kirkland
means "bright fame of church-land." Visualize Center is one kind of a
church. If Stiles can be considered as the eye of the Visualize symbol, then Red
John could be the bright around the eye. Kirkland always wears ties which are
red or tiger striped with prominent red color. In the last scene of episode 16 seasons
5, when Patrick and Lisbon are walking toward the crime scene, before we see
Red John’s smiley face on the wall, we get a glimpse of Kirkland and his red
tie and the unpleasant soundtrack increases a little bit. In the episode “Flame
Red” (S01E09) Patrick said to the killer Tommy Olds (whose voice by the way are
quite similar to the voice of Red John) that “only an idiot would wear a
t-shirt

like that
to a house in mourning ... an idiot or a daring killer with a warped sense of
humor.” Well, the same thing could be said about Kirkland and his red tie on
the Lorelei’s murder scene.



Regarding
CBI Ron, his name means "ruler's counselor".



If you
reason by Ockham's razor, then Kirkland becomes the prime suspect.

I think it's Bob Kirkland because he killed Jason Lennon

I have known Bob Kirkland to be Red John for a long time. Fine, I
shouldn't say I "know" because it's just my theory, but I do know! Why?
Because Kirkland has been around for a long time. He is in homeland
security and has a lot of power. The main reason I suspect him, however,
is because once he passed by Jane in the CBI office, and Jane turned
and look at him and said, have we met? And Kirkland said no. And that
was it. Now, that scene has no point to it whatsoever to that episode or
later. That's when I first realized that Kirkland had to be Red John. This is long before Jane came up with his list.

Furthermore,
the other five suspects are not right. Bertram the CBI isn't that
smart; he was once suspected as a link to Red John during the time when
Jane shot the fake Red John, and Bertram didn't have a clue. Jane pulled
the wool over Ray Haffner without sweating at all. Reede Smith is just a
stupid FBI guy. McAllister is just a policeman with no significance; we
hardly ever see him, and you can be sure that we do see Red John off
and on in the episode.

Bret Stiles is the mostly likely next
suspect because he has lots of power too, but he can't be it because he
already has a big following. Like Jane says once, Bret couldn't hurt a
fly. (When he does, he doesn't do it himself, certainly.) He is just not the hands-on killer that Red John is.

Therefore, it makes total sense that Bob Kirkland is it. He is also handsome, meaning that he can easy sucker women into doing his bidding.

Therefore, I know it's Kirkland.