Red john is someone who is the less suspect .. I'm pretty sure of that ! Remember that every case Jane has closed , the killer usually is someone we didn't expect him/her to be the killer .. Also the writer let us to suspect some people for all cases but finally the killer is not one of these suspects!

So from this theory , I rule out Brett Partridge , Bret Stiles , Kristina Frye , J.J. Laroche and Robert Kirkland to be Red John !