I've enjoyed watching the theories since I discovered the site since the season 5 finale. 

Two things I noticed:

Red John likes to upstage Patrick. Many have pointed out in one episode Jane walks away and then pops back out to scare Brett. Later Red John does the same to Patrick. Then in the season finale Jane catches a "break" and Red John makes it well known that what he did wasn't cleaver, and then Red John goes to prove that he himself is cleaver. This leads me towards Brett, seeing how Patrick has belittled Brett since he first met him. Always undermining him. Even when Patrick came on TV to dismiss Red John which in turn triggered his families murder. Red John has always had to upstage Patrick.

At http://www.youtube.com/watch?feature=player_detailpage&v=sd5glnwOmzw#t=17s you catch a glimpse of Red John. His hands and wrist look to resemble Brett's physique. Although as he walks by, he doesn't appear to be as slender as Brett. Perhaps it is his clothing that skews that. To note though, his hands while holding the cup appear to be a closer match. This scene for me at least rules out Stiles as his hands are much larger. I'll have to look for clips of the others.