If we are to believe that Bruno Heller and the writers of The Mentalist had Red John and who he was picked out, and his entry into the series plotted out for more than just the past season then I believe it must be Ray Haffner.
If you assume they knew who Red John was and are not just picking a guest that was previously on the show and trying to whittle him into Red John, then Ray Haffner's story fits not only the clues we know about Red John but the personality as well.
If you go back and watch "Little Red Book" which is Ray Haffner's intro to the series and look at with the expectation he is Red John every thing he says and does takes on significance. Assuming Ray is Red John, who became increasingly obsessed with Patrick Jane and took the assignment as a chance to not only observe Jane but uses it as an attempt to befriend him and his team.
-Ray is very prominently shown shaking Jane's hand when he meets him
-Ray specifically tells Jane he wanted to work with him.
-Ray tells Jane very seriously 'I would have worked with you, given the chance" (could be a double meaning - as Red John and as Haffner to Jane)
-Ray happily accepts Van Pelt on his team (to the surprise of J.J. LaRoche) he is eager to work with all of the team.
-He tries to befriend/turn Cho, ostensibly as part of Haffner's team against Jane but again could be a double meaning. As Red John, is he looking for "weak" people in Jane's group or possible acolytes?
-He questions Cho on Jane- what do you thing of Jane? Do you trust him? is he smarter than Haffner? Cho? He is more interested in Jane than the case.
-He pats Cho on the shoulder as Red John did to Jane.
-Even LaRoche thinks Haffner is behaving oddly and asks if "everything is alright?" because he is being so accommodating to Jane and his team.
- He says to Jane "it's a nice little game you ran on Tork"
-He says to Jane "You wanted my attention, you got my attention but if you try to run another game on me or my people we're going to have a problem" spoken as Haffner but also applicable as Red John.
-He says to Jane "did you really think that was going to happen that I wasn't watching you every minute" (may explain how Red John knew the seven suspects. He has been doing surveillance on Jane for years. He knew every person Jane had met, maybe even has tapes. Haffner is the "surveillance expert."
-Bertram says to Haffner (ominously?) "I thought you could handle Jane" and "Dont worry we'll find something else for you" and Haffner smiles (kind of odd considering he is being effectively fired)
-Haffner is a smart man who has worked his way up the ranks but he falls for all of Jane's tricks? He is obviously taking the job to observe them all, then let Lisbon have her job back now his "surveillance" is done.
-After his time with the team, Jane (and after Sam Bosco and Rebecca) he becomes interested in Lisbon. (because he knows she is important to Jane?)
-He tries to "take" Lisbon away by offering her a job or just testing her loyalty to Jane?
-Red John is supposed to be very convincing and hypnotic. Ray gives a very compelling speech to Lisbon asking if she is really attached to Jane not the work. His argument about her being alone with the work clearly strikes a chord with Lisbon. He has accurately read her desires and her fears regarding Jane. He is charming, empathetic and flattering with her.
-He is consistently good-natured and the only suspect of the five left who is believably charming and irresistible to women like Lorelei, Rebecca etc.
- He says "if there's one thing I've learned about Jane it's that catching bad guys is a game to him"
-He tells Lisbon "You're missing out on something big"
-The obvious clues mentioned everywhere, age and a member of Visualize.
-Lorelei his acolyte mentions how Jane is "a little bit in love" with Lisbon- did she get this from Haffner/Red John?
-Lisbon's head is the price of Jane's loyalty to Red John. Haffner is one person who saw how important they are to each other.
-Red John sent Lorelei, a woman to Jane. Was it to alienate Lisbon.
-Ray's seeming interest in Lisbon, always telling her how much he likes her, making sure he is her 'friend."
-Even when he comes to speak to Lisbon about a case, he can't help asking about Jane. How did Jane pull off his latest trick? Then smiling.
Plus all of the behavior recently- Red John sparing Lisbon, claiming her with his mark, the creepy hospital behavior etc etc etc.
Out of anyone he has the best back story and the most interaction with Jane and the team. I think it will be disappointing if it turns out to be anyone else.
