After reading through the entire comment thread on Screen Rant regarding who the last cylon might be (http://screenrant.com/archives/who-is-the-last-cylon-on-battl-1592.html), I would like to post my own theory here:

The last cylon model - the fifth - is (or was, I should say): Lee Adama’s mother.  That would make Lee a "hybrid" human/cylon.  There was an allusion to this possibility during a conversation between Lee & his father:  Lee said something along the lines of "What if Zack was a cylon?" (in reference to how they should treat Kara, as there was reason to suspect she could be a cylon.)  Although that comment was said in passing, most allusions are intentionally non-obvious at first.

If Lee’s mom was a cylon, then both Lee and his brother would be cylons.

There is supporting evidence for this.  In an earlier season, Leoben whispered to Roslin that "Adama is a cylon".  Of course, he could have been lying - they made it clear that Leoben tells both truths and lies - but if it was a truth, then he could have been referring to either Bill Adama OR Lee Adama.

Failing the above theory, then I would posit a similar one:  Starbuck’s mother - or father - was the fifth "hidden" cylon model.  Thus Starbuck would be a cylon - which would also explain some of her behavior and her comment in the last episode about feeling like she’s watching herself from outside (something like that).

Lastly, let me say that I like the theory that all of humanity could be cylon.  Or that Earth is inhabited by cylons.   But I’m somewhat doubtful they will go this route.