I really think it's Walter Mashburn. He has the money and power to be Red John. He has a dynamic personality and is likeable. Red John has many friends and none of the other suspects seem to have it in them to draw people to them. I can't see Red John being one of the boring characters that always wears gray. I see him wearing red and having an interesting personality.
One last point: it is going to look very unrealistic is Red John does not have money and power from a legitmate source.
