It took me a while to write this whole theory so please read before upvoting/downvoting.
We know, from an episode in season 4, when Roy pays Rosalind a visit and kills the morgue attendant to prove he's alive, that he wears a ring in his left hand: http://i.imgur.com/UZGYhAl.jpg (that pic is a little blurry, but if you see the episode in HD, you'll definitely see it).
Brett Partridge, the most likely suspect, doesn't. Here's him in episode S05E15, Red Lacquer and Nail Polish: http://i.imgur.com/erDT36y.png
As for Kirkland, he doesn't wear a ring either, and he holds his drink with his right hand: http://i.imgur.com/Emn4cbH.png
Haffner - no ring: http://i.imgur.com/YLC8HCc.jpg
Same for Stiles: http://i.imgur.com/e7uvgso.png
And finally, Sheriff McAllister, no ring either: http://i.imgur.com/b8KhrTg.jpg
As for Smith, I couldn't find a video where he shows his left hand, but come on, the guy doesn't fit the RJ profile at all. Red John has a rather slim figure (not soft but not beefcake, as described by Rosalind Harker) and, according to the writers, his real face has been visible in episodes before season 4 premiered. This guy first appeared on S05E01 and I'm not sure if he even had any lines. I think it's safe to rule him out.
Bertram, on the other hand (well, not literally) DOES wear a ring: http://i.imgur.com/ifJRtcr.png
If the list can be trusted (and Heller isn't screwing with us), then, judging Roy Tagliaferro's appearance, Bertram is Red John. There's also him calling Patrick "boy wonder" in S03E01, his William Blake quote, the mall scene in Strawberries in Cream (how else did the fake Red John know he had to be at that particular mall at that exact time?), his resemblance to the mysterious police car driver in S04E24, etc.
There's however, other things that don't match. He's over 6 feet tall and he's mostly bald (according to Harker, Roy has short hair). The rest fits, though.
