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.