I say it's Rigsby just because he's the most "Bruno Heller would pick him" of suspects (least expected & completely out of left field) other than Partridge not being dead.  Plus he & Van Pelt are leaving the series, and given Van Pelt's success with men, Rigsby being RJ is the icing on the horrible cake of Van Pelt's love life, which leads to her exit from the show to "put herself back together again" after the discovery & death of Rigsby/RJ.