Pros:

• I can’t think of a single one…

Cons:

• Yuck
• Since she died on New Caprica she would have resurrected if she were a cylon.  Since the cylons don’t know who the final cylon is, it’s impossible for her to be a cylon.

Overall:

Why would the cylons bother?

Chances of being a cylon: 0.0000000000000000000001%

Gaius Baltar - Genius, selfish, traitor and now self-proclaimed prophet

Pros:

• He sees Six in his head.  Maybe he is nuts… maybe he has a deeper link… perhaps a Cylon link?
• He betrayed all of mankind.
• His only friends are cylons.  He’s a loner.  Could be a perfect sleeper agent.
• Three of the final four cylons have been noticed in the crowd listening to his preaching.  Why?
• He is one bringing the news of the cylon god to the humans.

Cons:

• We were down this road in season 3.  Baltar thought he was a cylon and discovered that he wasn’t.  Revisiting that wouldn’t be the big "reveal".
• If he were a cylon agent, they wouldn’t need to send Six to seduce him.  They could just activate him.
• He lived on a Basestar for a while with the cylons… and the cylons claim to not know who the final cylon models are.  It seems unlikely that he could live on the Basestar for that long and they wouldn’t determine he was one of the final models.
• Six stated that she wished to have a child with Baltar.  We’ve seen that cylons require a human to reproduce.  Two cylons cannot have a child together.

Overall:

There’s more to Gaius Baltar than we think.  In the end, Baltar might turn out to be more than just a self-absorbed jerk.  However, it just isn’t feasible that he would be the last cylon model.  Baltar seeks redemption and, well, he needs it.  But will he seek redemption with the humans or with the cylons… or will HE be the one that unites both together?  I doubt very much that he is a cylon, but if he is… he will end up being the chief cylon.

Chances of Being a Cylon: 1%

Pros:

• The irony of having the person who played Apollo in the old show now be a cylon is pretty cool.  At first Richard Hatch was upset on the new direction with BSG, but the producers reached out to him and brought him on the show.  I wonder if they enticed him by making him a cylon…  That’s a pretty interesting concept.
• He’s a loner.  No family ties that I can think of.
• He was a minor part of the resistance.  3 of 4 of the last revealed cylons were part of the resistance on New Caprica.  It would be consistent.

Cons:

• Is he too minor of a character to be the final cyon? 

Overall:

Tom Zarek could very well be the final cylon.  However, I just don’t think it would be the "Oh wow" moment we want the show to give us when the final cylon is revealed.

Chances of being a cylon: 5%

Pros:

• It would be a great "oh wow!" moment.
• Having a captain (and now admiral) in the Colonial fleet would be a killing blow to the humans.
• It could explain the close bond between Tigh and Adama…

Cons:

• If you are going to infiltrate the Galactica with Tigh, it doesn’t make sense to add Adama as well.  One would be enough.  It would especially be counter-intuitive if the cylons didn’t know they were cylons.  Adama would be on the lookout for cylons as well as Tigh and they could accidentally stop each other.
• He’s just too old.  He fought in the first war before the cylons had mastered creating human-looking cylons.  This would mean that he would need to be a human to start with and then replaced down the road.  Again, the cylons haven’t demonstrated that ability.
• He’s the father of Lee Adama.  Hera is supposed to be the first cylon/human child, but that would make Lee the first.

Overall:

While it would be a great reveal… that the Admiral of the fleet is a cylon… it just cannot work.  He’s too old and doen’t fit the profile.

Chances of being a Cylon: 2%

Pros:

• From going AWOL to defending Baltar to leaving the service, he has probably caused more chaos than any other human on the show save Baltar.
• He’s wandered.  He’s gone from viper pilot to Captain to lawyer to politician.  Almost like he is looking to find where he is supposed to be before awakening. 

Cons:

• Since he has family relation (his father) that has watched him grow up, the only way he could be a cylon is if he were swapped out at some point – that he really wasn’t always a cylon and at some point was replaced with a cylon.  This doesn’t seem to be the MO cylons use.  They haven’t shown that cylons can copy a human exactly and replace them with a copy with all of the memories intact.

Overall:

The only thing blander than Lee’s personality is lukewarm water.  Wait… no, that’s more exciting than this guy.  Maybe he is supposed to be the show’s “conscience”, but his whining is unbecoming of a cylon.  With the family ties, it seems almost impossible that he could be a cylon without someone noticing the switch.

Chances of being a Cylon: 2%

Pros:

• She was Secretary of Education before the cylon attack.  Is there a better way to influence another culture than through education?
• She lies, cheats, steals whatever to do what she thinks is the right outcome.  Isn’t that a cylon trait?
• She has visions.  Perhaps they are cylon messages coming through?

Cons:

• She was dying of cancer and then healed by the half-cylon/half human baby’s blood.  It would be very doubtful that cylon blood would help her out.
• She’s out of child bearing age.  That doesn’t seem to fit the rest of the female cylon pattern.
• She wears glasses.  Now, that’s not much of a big deal, but none of the other cylons have any physical defects.

Overall:

Roslin would be a prime suspect were it not for her health issues.  She’s in a prime position to really put the hurt on the humans.

Chances of being a Cylon: 8%

Pros:

• Not much is known about Helo’s history.  He apparently doesn’t have much of a family history so probably could easily be a cylon.

Cons:

• He had a baby with Sharon, a cylon and that kid has been referred to by the cylons as "the hybrid".
• He has been very cylon sympathetic.  The last 4 revealed cylons were all very anti-cylon before they became aware.
• The last 4 revealed cylons all resettled on New Caprica, Helo stayed onboard the Galactica.  It’s not a show-stopper, but it doesn’t match the rest.

Overall:

Cylons don’t reproduce sexually, so since Helo had a baby with a cylon, it would be a one in a million shot that Helo is a cylon.  The cylons have been infiltrating the humans and matching up a cylon with another cylon doesn’t seem to be a good infiltration tactic.

Chances of being a Cylon: 1%

Lt. Gaeta - tactical officer on the Battlestar Galactica.  Righthand man to Baltar on New Caprica. Pros: He was part of the resistance movement.  We know that 3 of the 4 that have been revealed were part of the resistance movement on New Caprica.  Tigh, Anders & Tryol.  Why the hidden cylons were part of the resistance, I have no idea, but it appears to be a common thread. You wouldn’t expect him.  In fact, you probably haven’t even thought about him. Who better to have as a plant on your side than the tactical officer in the enemy fleet? He rerouted firewalls pretty fast when the cylons were trying to hack into the Galactica.  Maybe he let them in????

Cons:

• Would anyone care about Gaeta being a cylon?
• He is missing from the goofy image SciFi has on their website.

Overall:

Gaeta would be an excellent choice for a cylon.  He would be in a major position to cause complete chaos onboard the Galactica.  He’s under the radar and a loner.

Chances of being a Cylon: 38%

Pros:

• She seemingly died and then came back.  Something only cylons do…
• She seems to have a major destiny that the cylons actually know about.
• She’s nuts.  She has visions about where Earth is… it seems to be more like a cylon thing.

Cons:

• It’s too obvious.  They keep building it up that she might be a cylon.  The crew suspects it and the audience suspects it.  That’s why it wouldn’t work. 
• She’s married to Anders who is a cylon.  The cylons seem to be teamed up with humans with the intention of procreation.

Overall:

We’re being led to believe that Starbuck is a cylon, but it would be a complete disaster if she really was.  We all see it a mile away and it wouldn’t be the big "reveal" or "surprise" if she was.  The issue of how she "died" and now is back could possibly be explained by her "destiny".  The new series parallels the old series in many ways and in the old series episode "War of the Gods", angels resurrect Apollo… and with the religious overtones of the show, I think we’re going to see something like that… that angels have resurrected Starbuck and not because she was a cylon.

Chances of being a Cylon: 5%