I am not going to accuse anyone for sure yet, but here are my suspects:

1. BRETT PARTRIDGE

-Fits all RJ characteistics. (Age questionable)
-Is in Jane's list of people he shook hands with.
-Has a "serial killer vibe " in my opinion
-He has a nearly identical voice to RJ
-According to another theory I've read, his opening line is "Red John enters here." (refering to a case, but that would still be clever)

2. ELLIS MARS

-Deceptive abilities (Psychic)
-"He is Mar" written on a wall, probably incomplete.
-In Jane's book (see Brett Partridge, line 2).
-Not very much background with Jane that we know of, lessening his chances of being RJ.

These are my main suspects. Again, I have not officially accused anyone, yet.

~Red John