I am not sure who Red John could be, but thinking about a comment I recently read, I had a crazy idea come up:
when Kirkland asks Lennon, if he recognizes him, could it be that he had plastic surgery? Lennon knew the old RJ face and if he wouldn't recognize him now, RJ can be sure that his disguise ist perfect.

I don't like the idea too much, but it the only way that Kirklands question makes sense to me so far.

So, if Kirkland had plastic surgery, how has he been looking before? Compare Kirkland to Walter Mashburn... face would be possible, the only thing is that Walter is taller than RK... that doesn't fit. But Teresas role would be explained. RK has been to PJ's room, so that could hint to the LM video.
So, what do you think?

I always thought that RK is NOT RJ, bur the more I think about his behaviour, the more plausible it seems to me.
I still like the idea of Walter being RJ, it would be the most interresting character to be RJ as PJ has a good connection to WM and also WM has a crush on Teresa.

All the other characters would be disappointing to me. 
Partridge is a possibility, too. He's has been one of my main suspects all the time
Stiles - I don't think so
Reede Smith - I don't think so
Gale Betram - possible, but I don't think so.
MCAllister  maybe, strange character...
Ray Haffner - I reall don't think it is him