Writers have compared RJK to Waldo, a famous character for children who appears in the background. "You've got to look at it like Waldo" as Ashley Gable once tweeted in the offical Mentalist Writer twitter account.

That being said, characters like Brett Stiles or Walter Mashburn couldn't possibly be Red John. They're not background characters, they're simply "out there" and are target of suspicion to lots of people. It's almost too obvious. Same thing goes for other characters who have a strong presence in the show, such as Bertram or Minelli. Personally, I would rule out anyone who is highly suspected by a large number of people.

On an unrelated note, I also think it's unlikely that Red John is someone we've only seen once in the show, such as Dr. Royston Daniel, simply because it would be too disappointing. 

Red John can't someone of too strong presence in the show, but neither a random guy we've seen in one single case.