I perceive Red John as a subtle, calm, patient, cautious man, and maniac in the way that detail is very important to him.
I would be disappointed in finding out that Partridge is in fact RJ, because he's seems to be a simple-minded fellow. And the way he jumped in to defend the killer when Jane said it clearly couldn't have been RJ proves to me that he is an unlikely candidate.
Mashburn... eh. No. Why would two women of the show be sleeping with RJ and his affiliates?
Bret Stiles? Doesn't seem likely to me. I can't defend myself on this point, but it wouldn't surprise us enough, I think.
"Hiding in plain sight", I interpret it more like this - we see him, someone may have already spoken to him a few times, but I don't think he's been major and present all along.
But that's just my theory, I'll remain humble.