I Love the Mentalist - have watched pretty much every episode but I really have no idea who Red John is and it is eating away at me!!
I think there are several questionable people - LaRoche, Rosalind Harker, Kristina Fry, Susan Darcy...to name but a few! I dont think it can be PJ. Just think it would be a poor ending and it just seems impossible given everything that has happened in various episodes. I think Red John actually models himself a bit on PJ however - drinks tea, sits the same, obviously likes to be in control and does things on his/her terms. I think Patrick may end up being framed due to the similarities and have to perform a bit of his customary magic to get out of it and discover the real Red John.
I tend to like to concentrate on a few key episodes - The Ep where we see RJ in a mask reciting the poem is obviously important, as is the conversation that Kristina Fry has with the waiter where he says Roll Tide - as RJ does at the end to PJ. Also, the episodes with Rosalind are obviously crucial as she knows RJ intimately and in season 4 where she doesnt know there is a body in her closet she seems so agitated when the CBI/FBI arrive. I also think that the upcoming ep S5 Ep5 is going to be clutch - why would they go back and show how Patrick first came into touch with Lisbon and her crew if it wasnt crucial?? So much to ponder, yet still no closer to having a formed opinion as to the identity of RJ - I guess thats why I love the show so much!!

PS. Has anyone else ever noticed how often Patrick taps people on the shoulder twice - other notable characters have also done it and the people they touch often end up dead/missing. I think it may be a little something that could be important. Keep a lookout for it and let me know if anyone else has noticed also!!