Ok, first off, spoiler alert for the first episode of season 4, action stations! So this week it looks like we're going to learn who the last Cylon is, well the commercial promises "all will be revealed" at least. I'm going to take that too mean we learn who the twelfth and final human Cylon is and have some theories...

Who I don't think is a Cylon:

Gaius because he represents what man will to his fellow man in his own self-interest. We don't know if he feels any guilt for causing the near extinction of his own species, not once, but twice. Caprica Six would also be free of the guilt It would also ruin the whole thing with Six being inside his head while Caprica has a version of Baltar in hers as equal and opposite.

Bill Adama or Laura Roslyn because they are the mother and father figures and stand in opposition to Gaius because they've put the survival of the human ahead of their own lives. They both believe in the rule of law, they've also subverted the law when they believed they had to and the justified the means with the ends. It wasn't that long ago Bill had thrown the president in the brig and she tried to rig an election. Bill challenges us to be better, "it's not enough to survive, we never proved we deserved to" and while the president struggles with internal and external religious conflicts. All very human stuff.

Helo shows us that love can conquer. He found out she was a Cylon and pregnant at the same time shortly after her people launched a nuclear holocaust on his people. And now they're married with a baby girl.

D or Gaeta, cause really?

Cally for a couple reasons. Would ruin the drama if it turned out both she and the Chief are Cylons. He's now got a secret he can't tell her and makes him different from her.

Kara for much the same reasons but with Anders, minus a kid. I think they're setting this up so she will be accused of being a Cylon for a complete mind-frak on everyone: Anders is already freaked out, and now that Lee is not in the military they have to make him useful with a reason for being on Galactica and being either her defense attorney or the prosecutor, either of which will be awkward. Adama will probably feel he's been betrayed by a woman he considers his daughter, and/or his son, and/or President Roslyn depending on her roll in all of it.

So that leaves:

Lee. Who could possibly be more shocking to Adama, Roslyn or Kara? He's been a moral compass for the president and his father. Everyone has repeatedly placed their lives in his hands and he's come through. Last week Lee asked his father a question, if his brother Zak still alive, but turned out to be a Cylon would it change anything? Well, what if Adama was faced with his son being a Cylon? Anders asked Kara the same question of himself and she said she'd put a bullet through his brain. Would she feel that way if she found out Lee was a Cylon?