RJ has to be someone who has the following access.
computers, media, phone records, big money, time, criminals aka vice especially, pharmeceticals, medical background, political influence,

Bret Styles has access to all the above however, like PJ said " you wouldn't hurt a fly". 

 I think he (Bret) has a pretty good idea of who RJ actually is, but is afraid of him or it's not profitable to him to turn him in. I do think Bret has some modicum of affection for PJ which is why he helped him with Kristina.
They always show RJ with cowboy boots, well made pants, mirroring PJ i.e. cups of tea, lock picking skills, obscure literary references, and generally running PJ through pre-designed mazes. He clearly has access to CBI privileged information, is methodical, and his pride is what will cause him to make "the mistake" that will let PJ catch him. He has access to private information such as addresses which aren't available to general public, my guess is he's in the higher sections of the government. Probaby based in California, because people only watch the news in their local area, so he had to see them on a local channel, access to guns that can't be traced, local because he is definitely driving to some of these places, . although a rundown of airplane passengers int and out of these cities might provide some clues to identity unless he's flyng his own plane. Even then,, he would have to file a flight plan..

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!!

He has the ability to influence people and people to commit the crimes. Series 4, Episode 16  (His Thoughts Were Red Thoughts) gave lots of clues, particularly the name. Bret talks about seeking happiness and blowing "red smoke" to release anger. He mentions information at the end of the episode that only the FBI would know and has an obvious need to compete with Jane in mind games. 

An easy pick for Red John.