Wouldnt it be a disappointment if Red John was Brett Partridge, given that he has the look and the voice to match Red John.   It would be too easy and predictable.  If Red John were Rigsby or even better yet Cho, now that would be a fun story as it plays out.  As for who my guess is, I like the posting earlier today about Amos Van Pelt and Grace being his accomplice, but who knows.