Great series, but the last seasons were awful. FBI? Can't suspend disbelief on that.
If I were to pick a number one Red John candidate, then it would have to be Rigsby. His father was a biker dirt bag, he fell in love with the redhead, he says things that are out of character, and he is, like someone else said, a classically trained actor. There could be no better choice!
If one were to watch the episode where he is on an island with the cbi team solving a murder, then one could see how Rigsby could be a great Red John.