This post isn't it, by the way. (So let's not speculate on who the final cylon is in the combox; let's save that for next time.)
But I thought I would do a little bit of speculating about who is a cylon in this post, and I think that the woman on the left is.
Who is she?
Well, it's Ellen Tigh, wife of Col. Saul Tigh.
Why do I think she's a cylon? A big reason is that there was a lot of hinting in the very first episode she appeared in that she is a cylon.
Remember? She was found mysteriously on one of the ships of the ragtag fleet, unconscious, with nobody remembering how she got there. Then she wakes up and is brought to the Galactica, where she proceeds to bring out all the worst aspects of her husband, like a cylon sleeper agent might want to do to undermine fleet command.
And to settle the question of whether she is a cylon, Dr. Baltar gives her his cylon-detector test and publicly says that she's a human but privately admits to Head Six that the test may have indicated otherwise but that he'll "never tell" if it did.
So there's all that.
It later became clear that, if Ellen was a cylon, she wasn't a sleeper agent who knew what she was doing, because she was willing to risk everything to help her husband during the cylon occupation of New Caprica. Or at least she didn't care about betraying cylons to help her husband.
But that's not a problem since not all cylon sleeper agents know that they're cylons. The original Sharon "Boomer" Valerii, for example, did not know she was a cylon and others, like Sharon "Athena" Valerii have sided with humans against cylons.
So things were looking pretty inconclusive regarding Ellen, until . . .
"Wait!" you say, "I haven't seen any season 4 episodes! Don't spoil this for me!"
Stop reading now.
"Wait!" you say, "How can you say this post isn't going to be about who the final cylon is if you're about to say that Ellen Tigh is a cylon? Wouldn't her being a cylon mean that she's the final cylon?"
No, because I think she's just a version of one of the cylons that has already been established.
Specifically: I think she's a Six.
She's just an older version of Six--perhaps one of the first Sixes to be made.
After all, the Sixes don't all look the same. Even when they're played by Tricia Helfer, they look different. Some, like the "Gina" Six (the one who was on the Pegasus and later set off the nuke) or the current "Natalie" Six (the one leading the cylon resistance to the Ones, Fours, and Fives) wear their hair notably different than the platinum blond curls of Caprica Six or Head Six (and Head Six herself has worn her hair differently, as she did in the episode where she got Baltar to get the brainscan that convinced him she wasn't a chip in his wetware).
So Sixes don't all have to have an identical physical appearance, and thus there might be older versions of some models that have aged in a way that simulates human aging.
But that's not the core reason.
The core reason is that in the recent episode "Escape Velocity," Col. Tigh interrogates Caprica Six in her cell on the Galactica and, during the course of the event, starts hallucinating that Caprica Six is Ellen.
What they did was bring back the actress who plays Ellen and got her dressed up and coiffed as Caprica Six, platinum blond curls and all.
And, wow, is it convincing.
There are moments, watching the transitions between Six and Ellen, when you say "Which actress am I looking at here? They're so close once you put the same hair on them!"
And then there's this . . .
The characters act the same.
They're both highly manipulative and willing to use their feminine wiles to achieve their ends.
Also, neither seems to mind the concept of extra-marital affairs as long as some kind of fundamental love/loyalty-to-one-spouse isn't compromised.
And--perhaps most uniquely--they're both manipulating their Significant Others to make something more of themselves in the world.
I mean, think about it: From the moment Head Six shows up, she starts manipulating Gaius Baltar to ensure that he assumes his "destiny," right?
And from the moment Ellen Tigh shows up, she starts manipulating Saul to get him to assume command of the fleet/increase his influence and power, right?
Both of them are catty, morally loose manipulators who are trying to drive their chosen men reluctantly into positions of prominence and power.
So, not only do they look the same, they act the same. They have the same basic personality and modus operandi.
Okay, that's it.
Unless the creators of the show choose to establish otherwise on screen, as far as I'm concerned, Ellen Tigh is simply a more mature Six.
Which makes her story all the more interesting.