I have two theories on who is a Cylon. Let's look at who the Cylons we know of:
"Significant Seven"
Brother Cavil: Male
Leoben Conoy: Male
Simon: Male
Number 5: Male
Number 3: Female
Number 6: Female
Number 8: Female
"Final Five"
Saul Tigh: Male
Sam Anders: Male
Galen Tyrol: Male
Tory Foster: Female
?
So, of eleven known Humanoid Cylons, it is established that there are seven males and four females. I'd find it likely that the missing "Final Five" member might well be female, but of course this isn't assured by any means.
"Significant Seven" Cylons are unable to recognize a "Final Five", for reasons as yet unknown. Cavil did not seem to recognize Tigh was a Cylon during his imprisonment and torture on New Caprica. Boomer didn't seem to know Tyrol was a Cylon and they were physically intimate with each other. Of course, at the time she didn't even know she was a Cylon herself.
Of the four known "Final Five", ALL were on the surface of New Caprica during the occupation. It's possible that the fifth was as well.
Galen Tyrol, a known "Final Five", had a child with Cally. This means Cally is NOT a Cylon, since it was established that two Cylons could not have a child together.
Saul Tigh has a 'history' going back 40 years, to the first Cylon War. It is possible that the missing member of the "Final Five" also has a history exceeding what we've come to expect.
The four known "Final Five" all heard "All Along The Watchtower" and were compelled to gather in a secured area, where they all realized their true nature. This makes me think the missing Cylon was NOT on Galactica at that time, possibly not even anywhere in the fleet. Possibly even dead.
So, who "fits" with the above and could quite possibly be our missing "Final Five" member?
ELLEN TIGH
Think about it. It actually makes a lot of sense. She even passed Baltar's Cylon Detector, which the good Doctor had fidled with so that everyone passes. Childless, since no Cylon Couple could bear children. Ellen and Saul together could have supported each other's "backstory" when they arrived 40 years ago, and presumably forgot they were Cylons. Saul killed her on New Caprica, so of course she couldn't be around to hear the song and make her way to the other "Final Five" meeting.
So where is another of her model? On Earth of course. Hehe
My other theory is rather more grand. Two ideas led me to it. The first, is that EVERYTHING Leoben Conoy learned from the Hybrids and repeated to Kara, Roslin, etc, is true. The second is "This has all happened before, and will happen again".
EVERYONE IS A CYLON
Humanity died out millenia before. A grand cycle repeats itself over and over. 12 Humanoid Cylons seed a world, and are the 12 gods of the pre-Colonial civilization. This civilization (Kobol) rises and falls, and ultimately 12 "tribes" leave to colonize 12 other worlds, where they ultimately develop robotic servants that rebel against them. These robots, after a long war and armistice, form their own civilization. Five of the original Cylons remain with these Colonies, and seven of the original Cylons go to join with the robots and create a new Cylon race. The cycle repeats, over and over, until a reunited Cylon Hybrid species manages to make its way to Earth, where the lost "Thirteenth Tribe" of 'true' humans went to, at the onset of the first cycle. The cycle repeats over and over, because the 'new' Cylons and the 'Colonials' are unable to reconcile to the fact that they are, indeed, all Cylons, and so fail to make it to Earth.
Think about that.
Of course, BSG could just as easily end with Saul Tigh coming out of the shower after Ellen wakes up from dreaming the last four years...