I'd always thought it was Stiles until a few episodes ago when I switched
to Haffner.  I thought of a good reason though why it can't be Stiles. 
He is a famous cult leader, how could he have gone to the psychiatrist
with a different name without her recognizing him?

They're
showing way too much happy time between Van Pelt and Rigsby, which can
only mean one thing.  They're going to get slaughtered by Red John.  I
would love to see Cho then go nuts and kill Red John after PJ finds him.