I believe this week's season finale will be a major game changer. We have all loved speculating on RJ's identity throughout the years and names such as Mashburn, Minelli, Bertram, LaRoche etc. have circulated constantly.
The finale will whittle the suspect list down to 7 and I believe a lot of popular names will be ruled out.
Drama and intrigue would probably dictate that RJ has to be someone who has been in the show since early on and has appeared multiple times. I don't believe that to be the case necessarily. Bruno Heller plans at least one more season and I believe this will be used to investigate fresh leads and explore more characters.
Let's look at the evidence over dramatic convenience. There are 4 major clues.
 The priest in Red Barn implies he saw a kid drawing the RJ symbol on the barn back in 1988. A kid seems to indicate anything from 5 to 25 so RJ would seem to be between 30 and 50.
Rosalind Harker describes him as  just under 6 foot (therefore probably between 5'9 and 6),  average build and  has short, straight hair.
Now let's go through the suspect list posted on the CBS website to start us off and see if they disqualify on the aforementioned criteria.
 BETWEEN 30 AND 50 -
This rules out the following for being too old - Bret Styles (actor is 69), Asher Maclean (actor is 55), Judge Manchester (actor is 65), Sheriff McAllister (actor is 57), Jason Cooper (actor is 59), Dr. Linus Wagner (actor is 55), Virgil Minelli (actor is 65), Max Winter (actor is 55) and Vint Molinari (actor is 62).
The actors that play Donny Culpepper, Ellis Mars and JJ LaRoche are all 52 so I've kept them in consideration as they aren't far off the maximum age.
 JUST UNDER 6 FOOT TALL (BETWEEN 5'9 AND 6'0) -
This rules out the following for being too tall - Gale Bertram (actor is 6'3), Doc Dugan (actor is 6'3), Gabe Mancini (actor is 6'2), Walter Mashburn (actor is 6'3), Reede Smith (actor is 6'3).
This rules out the following for being too short - Markham Shankar (unsure of actor height but appeared much shorter than Patrick Jane who is 5'10), Tolman Bunting (actor is 5'7) and Osvaldo Ardilles (unsure on height but looks no more than 5'5 compared to other characters).
I wasn't sure on the height of the actor that plays Glen/Glennda Snow.
The actors that play Ray Haffner and Richard Haibach are 6'1. The actor playing Don Henderson is 5'8. As they are only an inch above or below my minimum and maximum heights, I'm keeping these 3 as suspects.
 AVERAGE BUILD -
This rules out JJ LaRoche and Don Henderson for being overweight or overly stocky. I'm also ruling out Paul Fricke for being too slight. Glen/Glennda Snow appears way too muscular.
 SHORT, STRAIGHT HAIR -
This is quite inconclusive. Roy Tagliaferro (Red John) could have shaved, lost or grown his hair since he and Rosalind Harker were lovers.
However, I'm allowing this to rule out Dean Harker and Tommy Volker as their hair is much too naturally wavy.
I'm going to rule out Donny Culpepper due to the fact he is only a petty thief and, as stated above, is a little too old.
So... who do we have left?
Those who perfectly match the criteria are -
Dr. Royston Daniel
Through minor exceptions in criteria, I am still allowing the following onto my suspect list -
I believe also that the CBS website may have omitted a suspect or two in order to preserve dramatic tension. Coupled with the recent spoiler that Lisbon will be shocked when she hears the first of the suspect names, I believe Patrick Jane's other suspect will be her brother Tommy.
My final 7 suspects (that I believe will be revealed in the finale) are Robert Kirkland, Brett Partridge, Dr. Royston Daniel, Ray Haffner, Ellis Mars, Richard Haibach and Tommy Lisbon.
Partridge is scheduled to appear in the finale. I can see him being eliminated from suspicion dramatically at the end of the episode; bringing the suspect list down to 6 in time for season 6.
Kirkland will likely be revealed as either an RJ accomplice or, more intriguingly, someone like Patrick who is trying to get vengeance on the infamous serial killer. He's clearly a red herring (pun intended).
Haffner's Visualise connection will likely be further explored in season 6 and this will be another opportunity to bring back Bret Styles. However, I believe his plot arc will be used to finally bring down Visualise rather than reveal RJ.
Ellis Mars will probably reappear in season 6 but I can actually see him being a victim this time.
Haibach's been left a little open ended. It was clear he was up to no good as he was caught taking photographs of young women. I think he will be brought to justice for stalking in a future episode but won't be Red John. Maybe an accomplice though.
Tommy Lisbon's inclusion on the suspect list will be used to highlight a little more conflict between Patrick Jane and Teresa Lisbon. I can imagine Red John's Rules will have a video recording of Lorelai being forced to communicate RJ's new rules. This will involve threatening Lisbon's life if Jane doesn't do something that RJ wants. I can see this leading to Tommy trying to save his sister and getting killed by Red John in the process.
That leaves us with Dr. Royston Daniel. The perfect suspect. He's appeared early on in the show. He's charismatic, handsome, powerful and a master of hypnosis and NLP. An ideal adversary for Patrick Jane. They seem also to be the exact same height and somewhat similar looking (though Daniel has dark hair and Jane has light hair - nice symbolism). I can see this creating a powerful visual for their final confrontation.