Firstly, known facts:

- RJ is a white male. "Red Rover, Red Rover" (s04e23)

- RJ has shaken hands with Jane; they are known to each other, but are not friends. (s05e08)

- RJ's name possibly starts with "Ma". Note that there are three people who fit this criteria - namely, Madeleine Hightower, Lorelei Martins and Walter Mashburn. Out of these three, only Mashburn is plausible. (s01e11)

- RJ is "just under 6 feet tall; not muscular, but not soft either; short, straight hair; a gentle voice; rough, strong hands; he smelled of pine and nails and earth; and he liked to listen to me play the piano (especially Bach—he admired his rigour)." (s01e23)

- RJ has ties with Visualise. (s03e03)

- RJ has a fairly high pitched voice. (s02e23)

- It is still not clear what RJ meant when he recited 'The Tyger' (William Blake) to Jane. One might possibly deduce that it's an attempt to state that they are similar. Which is held up by recent episodes.

Mashburn - He's in a very good position to be a plausible suspect. He has the money, the power. He fits the descriptions to almost a tee. He is intellectual, narcissistic and above all competitive. It has been noted additionally that he has had several wives, the third -- he additionally states that "his third wife was in to all that goop" when talking about Jane's past as a psychic. No evidence thus far illustrates that RJ's third victim was a believer in the paranormal. From what we see, Mashburn's intellect does NOT level with that of Jane's.

Stiles - Again, in a very good position. It is stated that he was a friend of Ronald Reagan. He has illustrated both a strong knowledge of Jane and a connection with Red John. However, intellectually speaking, he has nothing on Jane. It is clear that Visualise is his first priority. However, it is acknowledged that Red John is adept at manipulating people, as is Bret Stiles.

J.J. LaRoche - It's a terrible shame that LaRoche isn't a plausible suspect, at all. His recitation of William Blake's 'Cradle Song' is such a tempting hint that he is at least an accomplice, of some sort, of Red John.

Luther Wainwright - Not only is he too small, he's too inexperienced. Also, he's dead.

CBI Ron - Seen walking alongside Kirkland on the way to Lisbon's office, and then seen to trail Kirkland as he leaves the CBI. Possibly a coincidence, but one might assume that he is, at least, an RJ informant. His name, of course, is a startling abbreviation of Red John, which, again, may or may not be a coincidence. No serious period of interaction occurs between Ron and Jane. This probably excludes him as a suspect.

Brett Partridge - A 'geek' by all accounts. It's clear that his understanding of Red John is limited. He does, however, suit most of the criteria. His position allows him infrequent, yet important interactions with Jane. His supposedly poor understanding of Red John COULD be a façade. Additionally, he illustrates the insecurities which Red John is shown to hold. His profile is suitable.

Out of the list provided, Mashburn is probably the strongest candidate.