OK, let me get this straight first. My first choice is McAllister, as I've posted here before. And the chance of Smith being the Red John is this thin. And no, this has nothing to do with his appearance or attitude. This is simply a matter of good storytelling. You don't run 5 seasons worth of a manhunt and then introduce the real bad guy in just one episode in season 5. Nope, that's a cop-out writing.
On the other hand, people gloss Smith over way too easily. As this season shows, he's actually pretty ruthless, smart, and capable of extreme violence. Think of Saul Goodman--dude sleazes around a lot, but damn isn't he good at what he does. Smith is just like that. Kudos to Drew Powell for that scene when he fakes obliviousness to Kirkland. That's a stellar acting right there.