I think Helo is the last Cylon.

Starbuck is just way too obvious, and it's not Ron's way.
I didn't read all the other theories, but if Baltar is a Cylon, then why would they need to send Number 6 to convince him to give her the access codes to the Colonial defense systems (as seen in the first episode ever).

Helo saved the 5 sick Cylons in season 3, by making them suffocate, so that they won't infect the rest of the Cylons.

-Ronny