Red John is:

Dean Harken (attention to the anagram for the lover of Red John, Dear Karker - Rosalind)

or Dr. Royston Daniel

or Max Winter

or else, Ray Haffner (In the last episode, when haffner is in the elevator behind him appear the letters R. .... ED)

Heller must give a great twist on the show, or else soon will reveal a big surprise. Otherwise the show will be the biggest disappointment in the history of TV. I love The Mentalist, would be a shame. 

Excuse my English, is not my first language.