Process of elimination here:
Reede Smith - If this is Red John it's crap writing.  Some guy we've seen for about 8 seconds of screen time, overall would be a very poor reveal.
Brett Partridge - Too young in my opinion, plus he isn't in a position of power where he could easily give people the things they want (RJ's primary tactic for getting people to do what he wants).  
Tom McAllister - Again it's crap writing if it's him.  He hasn't been involved in the series since episode 2.
Ray Haffner - Again he seems young, not that powerful, etc.  He could have been faking it when Jane took his team apart, but to me he just doesn't have the screen presence of a criminal mastermind.

So that leaves:
Bob Kirkland, Bret Stiles, and Gale Bertram.

It could possibly be Bertram, and he would be my second choice behind Bret Stiles.  He could have faked needing help at poker and his position overseeing Jane would be ideal for watching Jane flounder (searching for the man whose office is feet away).  He just doesn't strike me as overly charismatic and able to attract a following easily.

Bob Kirland is the obvious suspect, which generally means it's not him.  My view is that Kirkland is a former disciple of Red John's who has never met the guy.  Someone who got tricked into doing something he now regrets and/or wants revenge for.  So now he is trying to find Red John by riding Jane's coat tails.    He changed his appearance when he left Red John's employ to protect himself.  The reason he killed Lennon is Lennon knew what he used to look like, which would have put him in jeopardy from both Red John and the authorities.  As for why he didn't ask Red John's identity, it may be that he did but that was off screen or it may be that he felt Lennon didn't actually know who Red John currently is.

Which leaves Bret Stiles.  A man of significant means and charm, having built a cult from the ground up.  His age makes it more likely that he could be out killing in 1988, his position in the cult makes it more likely that he was able to cover up those same murders by relocating his cult people (who were working on that farm).  The only real thing going against Bret Stiles is that he let Lorelai out of jail.  This would be a highly risky move for Red John to make, although he may have made it just because he had gotten bored with Jane's lack of progress.

Although realistically, whoever they make Red John it will be highly disappointing.  It's been building up for 5 seasons and no matter who they pick there are going to be a few very good reasons why it's stupid that he's Red John.  My personal view is that they didn't have a specific character in mind when they created the series and even if they did having drawn the reveal out over six seasons and several different writers, it becomes difficult to not have contradicted yourself at one point in time... or just making Red John some random character like Reede Smith (since so little is known about him there's no way to make a case for why it isn't him... other than he's kinda fat and Red John would not be fat).