I think the answer was located in the second season and the final cylon could be Tom Zarek, Ellen Tigh, kat or Billy. Baltar could be, but for me it is very hard to believe because his character has new direction to go - religion conflict. The war of Gods in this version of Galactica will (or is) be monotheism (Balthar and the Cylons) X polytheism (Roslin and the most humans). ETs or Light things, will destroy this magnificent serie.
My bet is Ellen Tigh.