Interesting theory on who the fifth cylon might be on Battle Star Galactica from Byrne Reese.
... let's just assume that Baltar is an allegory for Christ. Suppose that Baltar, a brilliant scientist, was so obsessed with his own immortality that he sought a way to clone himself. Cloning of course is "easy" - the challenge is in the transference of consciousness. But this is exactly what Baltar discovered how to do... The first cylon he developed was modeled in his own image: meant to be the vessel for his own consciousness when the time came.... Baltar knew he would be tested in unimaginable ways. To help him along the way he programmed into himself a guide, an angel of sorts to instill in him a faith in himself (God) and is so doing help him survive and fulfill his destiny. This theory even helps to explain how an image of Baltar, a.k.a. "Head Baltar" appears to Six.
Another theory...
http://screenrant.com/archives/who-is-the-last-cylon-on-battl-1592.html