I think RJ is Ray Haffner. Why? Simply cause I don't think any of the other ones fit the profile anymore.
Partridge is dead. Also, in season 1 on the fake RJ case, he seems to eager with the case and doesn't really believe PJ when he says it's a copycat. But mostly: he's dead.
Kirkland is also dead. He met PJ late in the show and shook his hand, the episode before Lorelei gets out of prison. Lorelei doesn't speak to RJ after that since she's with PJ - how would she even know they've shook hands then?
Bertram is too well-known to be able to sneak around like that. He's been on tv several times to explain cases - shouldn't Sophie Miller have recognized him when he saw her?
Stiles is too old and too sick.
Smith is too young and too fat. We've seen RJ's shadow - he is not a big man.
McCallister could be RJ. I've had a hard time choosing between him and Ray Haffner. It really could be him, but for now i believe in Ray. Why? Simply cause he tells PJ in episode 06x06 "I'm not Red John". Why would he deny it, if he soon's about to reveal himself (as we know he will in the upcoming episodes)? Makes no sence.

That's why i believe Ray Haffner is RJ!