It's Sheriff McAllister.  Here's my case:

1) Red John has a deep connection to the rural areas outside of the city.  Yes, he has been active in the city areas but it's not his normal stomping ground - look at season one finale which introduced both a friend and a lover of RJ, both of which were in a rural area.   And RJ was friends with that killer's dad, so RJ has deep roots in that area or community. 

2)  Rosaland mentions that RJ smells of "pine and earth" which seems to be answered because Jane thinks he was building a cabin or something...but, I think that's how he smells all the time because of where he lives and the life he leads.  His friends are from small towns - and so is his past...which leads us to...

3)   The barn where it all started.  Visualize is always the thing people mention when discussing RJ's first murder, but I actually think where it happened is important too. Another connection to the rural areas.  

4) The first time he met Jane they "shook hands" in a unique way that played up the idea of Jane being a false psychic...it's the perfect intro for the eventual villain...and could be used as a wrap around later when they meet for real.  

5)  Episode 2 dealt with a couple who kidnap and sexually assault young girls, just like the other couple from season 4 (I think) finale.  Also, it's possible the couple from episode 2 was being encouraged by RJ, but of course they died so...who knows...

6)  He's the suspect fewer people look at...why?  Because not much is known about him.  He doesn't really make much of an impression whenever he's on screen...which is odd since the actor who plays him is quite talented....maybe he's supposed to be bland?

7)  Most of the other RJ suspects are this season in serious nail biting episodes...but not Sheriff McAllister....he's practically in a romantic comedy...

again, we don't take him that seriously...perhaps it's by design?

In summery -- Red John has connection or works in law enforcement, plus he has a lot of friends  in the suburban and rural areas.   Sheriff McAllister. 

Ok, not a Red John theory, just a thought that seems more accurate about Michael Kirkland that some people just seem to ignore, if you think there's something off on my thought let me know please, thanks!

Michael seems to be dead, there is no way we've seen him on the show. (it's reeeeally unlikely, not impossible)

Robert Kirkland has been searching for his brother for a long time, right?
He is a Homeland Security agent, that means, he stalks and get information about anyone he wants, just like he did with Jane.

Not put the pieces together, I hope you get my point, I mean, if someone else (Michael in case) was using his name to get access to the hospital and other things, there is no way he wouldn't know it.
And therefore there is no way he wouldn't go after Michael, his vendetta is all about his brother, so...unless he was lying about something, which I find truly stupid, since his vendetta would be completely pointless.
That's why I say: there is no way we've seen Michael on the show. If indeed that was Michael at some point, then, that seems a big script failure for me, just saying.
I think Heller put this twin thing just to twist our minds and it's working (again).

To everyone who discounts the old, hairless, fat folk:

Q: Who knows the most about possible RJ suspects?
Q: Who spent years developing his list of suspects, gathering evidence etc?
Q: Who is incredibly intelligent and a genius at seeing evidence, clues, reading people, deducing suspects?

Obviously i talk of Patrick.

Now, i know he doesnt know everything (such as Ray's whistling), but he knows all the evidence from Harker and Miller.

IF SOMEBODY MADE IT TO PJ'S FINAL LIST OF 7 THEN JANE THINKS THIS PERSON QUALIFIES, TAKING INTO ACCOUNT THE PHYSICAL DESCRIPTIONS.

Who are we to say Stiles is too old if Patrick doesnt think so.
Who are we to say Bertrum cant be Roy Tagliaferro, if jane thinks he could be.
Who are we to say Smith is too fat if Jane doesnt think so.

I know that the descriptions say short hair, just under 6 feet, in good shape, middle age ... and I realize this seems to us to eliminate some of the 7, but if Jane has whittled down a list of 1000s to only 7, if jane has been so judicious to exclude guys like Mashburn, then clearly jane has some plausible reason for these people to be the guys harker and miller were describing. I cant think of good ways to explain the physical differences, but i (nor any of us) need to turn our minds to this at all --- BECAUSE JANE IS CONVINCED THEY COULD BE RJ IN SPITE OF THE PHYSICAL AND AGE DESCRIPTIONS. If he isnt ruling them out, then we should not.

Just an interesting observation. Notice that while Kirkland was moving out of the van he did some sort of swallowing act i.e. He swallowed something that will pass some information to Jane.

Do you really think he wont be ready for this? Remember, he likes to cover his bases. He wouldn't just die without leaving a clue for Jane.

You know, with his whole "Tyger Tyger" organization that has been shown around this season. I think that Tommy Volker was involved in this organization. He was very powerful, and he had powerful political connections. Maybe that's why Kirkland told Lisbon to stop her investigation with Volker, because Homeland Security already suspected this secret organization.
Maybe Volker could provide some info for Jane and Lisbon about RJ during this season

What do you think?