Okay, so here is my theory!
Just to be clear, I (like everyone else) don't know who RJ is, but I've been thinking about a few things.

1: The "Tyger, Tyger" -thing. This is a poem by William Blake, god knows what it means when they all say that, but I have noticed that Bertram quotes Blake in several episodes. Can't remember season or episode-numbers, but I've noticed it. If that's a clue that it IS him, or that it isn't, who knows.

2. Partridge. He's always seemed very suspicious, and his death made me even more convinced it might be him. What doesn't add up to me is that i've always pictured RJ to be older. In season 1, we find out that the cop who dies in the end (after trying to kill PJ) knew who RJ was since "he was a friend of his fathers". That cop must have been atleast 25, so RJ should be 50 or so. Of course, his father could have befriended a younger man, but if i don't remember it wrong his father and RJ had known eachother for "a long time" and been in prison together?

3. Haffner. The way he reacts when he speaks to Lisbon in the coffee shop made me think it's not him. RJ is such a smart, "chilly" serial killer, it doesn't seem to be his "style" to just deny he is RJ, and he says to Lisbon "will you stop thinking it's me if i come" etc.

4. Kirkland. He is obviously a crazy, crazy man. I, like everybody else, have been thinking about the theory about his brother - does he have something to do with RJ? IS he RJ? And did Kirkland die for real? We still haven't seen any bodies...

I can't wait to find out who it is, just hope I'm not disappointed! The worst would be if RJ isn't a real person but just a conspiracy, that'd just SUCK.