First of all, sorry for mi English.

I've seen a lot of people who will be disappointed about Red John's identity, we must remember, he's just man, probably, one of the seven suspects on Jane's list. Mi theory, we have to forget about the rest and focus on that list, because Red John is there.

Before knowing the list I thought Red John was:
- Virgil Minelli
- Walter Mashburn
- Bret Stiles
- Brett Patridge 

So, as I said before, we can forget about Minelli and Mashburn. 

Bret Stiles is Red John:
- He has a lot of friends everywhere 
- He knows a lot of information about Red John, where is Kristina Frye for example. 
- He is intelligent, convincing and he likes tricking Jane.
- He is the leader of Visualize and we haven't real information about what it is, but who knows, maybe it's a place to find new "friends" or disciples. 

Bret Stiles is not Red John:
- He is too old (it's just an opinion)
- It would "too easy"

So, the common sense make me think that Bret Stiles should be Red John, but I don't know exactly why, I've got the feeling that it would be too easy for us. I mean, the first time I saw Bret Stiles, I thought about the possibility of being Red John. I think Bret Stiles knows that he is one of the suspects and he just wants to trick Jane. To sum up, in my opinion, Bret Stiles is not Red John.
 
Brett Patridge is Red John:
- He appears on the first episode ("Pilot")
- He always appears when someone imitates Red John. 
- He works for the police so he has access to the information he needs.
- He seems to admire Red John
- Jane hates him

Brett Patridge is not Red John:
- Some people say that he is too young (in fact, he is 44)

As you can see, I think Brett Patridge is Red John.
He appears on the first episode ("Pilot") and he is hidden until the last episode of the second season where someone else tries to imitate Red John, to imitate him. 
Why starting everything with an episode where someone imitates Red John? It would be better starting with a real Red John case. Why with an imitation? Because they wanted us to see Red John's real identity since the first episode, they wanted us to see Brett Patridge.

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.