Cally Tyrol ‘08! She’s more robotic than McCain, Obama or Hillary!
Here’s why it’s not some of the other characters:
I doubt it’s one of the secondary characters… by which I mean people like Dee, Zarek, the Doctor, or any of the pilots who seem to appear as secondary characters on an as-needed basis.
Those characters aren’t juicy enough to make the revelation of them being the 5th Cylon spectacular.
So let’s think big…
It’s not Gaius Baltar because the Cylons had him on board their ship, and could have determined whether he was one the five at that time. But he’s not. But he is morphing in to an evangelical, near-messianic figure who serves the Cylon god.
It’s not Kara Thrace (Starbuck), simply because she’s been treated like any other human by the Cylons, including the time they attempted to harvest her ovary.
It’s not President Roslin because she’s human enough to be afflicted with cancer, and if she were a Cylon, it’s doubtful the blood from the hybrid child (Hera) of Helo and Sharon (Athena) would have had such a profound (though admittedly temporary) effect.
This also eliminates Helo, since his humanity is why the Cylon’s manipulated him into mating with Sharon back when he was on Caprica.
I doubt that it’s either of the Adams, though theoretically, it could be Commander Adama or his son Lee Adama because, hey, if XO Tigh is a Cylon, why not Adama? But that would mean that Lee Adama is, at the very minimum, a Human/Cylon Hybrid… but if Lee is a hybrid, it would have to be the result of some kind of bizarre baby-swap/kidnapping by the Cylons a long, long time ago.
Not frackin’ likely.
Factor in the plot from today’s episode (I won’t spoil it for those who haven’t watched it yet), and yeah… it’s Cally.
And the beauty of it is that it would make her baby with Tyrol revolutionary because it would be the first time Cylons were able to breed without using humans to make a hybrid. 
P.S., I think Tory is the most underrated hottie on BSG. Check her out. Yum!
[...] (But I still wish it was Cally!) [...]