Editing User:Staf27TC5

Laura Roslin

It would seem to me that Laura Roslyn (The Master Manipulator or a modern day Christ) is the fifth and final Cylon. 
The reasons are as follows.

1. She's in the first season. (Main Clue)
She's the biggest protector of humanity. She puts the safety of the people who would populate her dead world above all, including the cylons themselves. She and her fellow final five would already know of the cylons evolutionary progress. (All this has happened before).

Subtext for that thought-
 All of the final five have been strident protectors of humanity knowing of it's demise from whence they came. This rules out Adama’s son Lee. The final five are the ones who actually built the temple on Kobol. Kobol would have actually been the start of the stone trail the left behind them.

Laura Roslyn assumes control of the fleet to assure that earth's destination is reached. (She has absolute control even the cylon fleet). She persuades Adama that the poeple must live and make babies, securing Adama's agreement of her prime mission (She's the one who suggested that they find earth). Since she's the "final cylon" she has control of the humans and the cylon.
  She has control consciously and subconsciously of the final four.
  She's already in control of the cylons even before her second layer of programming is activated. Note that in "The Hub"  [beautifully titled because that's exactly what she is, as Jesus was a hub to Christianities god] Another layer of Laura's programming again engages as Elosha appears to her while jumping. The hybrid is actually instructed by Roslyn to make those jumps to activate the next layer of programming.
  
- Every Church has a leader right (Laura Roslyn [Jesus]- teaching her disciples)- The number sixes can categorized as the the Jusuits of old (founded on earth with 60 members). The jusuits were originally founded on "For the greater glory of god". The jusuits were also the churches assassins.
Paul III confirmed the order through the bull Regimini militantis ecclesiae ("To the Government of the Church Militant"). Perfect description for number 6 and Laura Roslyn's control of the government. 

Laura's the most successful cyclon human of the final five...she has achieved near divinity because of her terminal human illness and her connections to god and her conscious and subconscious control of the final four (disciples). She herself would be the chosen one, gods daughter (so to speak). And of course she uses a number six to carry out all of her assassinations and plots.

Those include the timed destruction of the defense system on Caprica. Remember the show opened with Laura looking at a number 6 who was in control of Gaius. 
A number six kills Admiral Cain after Laura Roslyn instructed Adama to do it (he refused) in comes a number 6.
A number six strands triggers an explosion that would start stop the clock, allowing a purposely lost election to Gaius Baltar.
(the stars were not in alignment yet).
A number six manipulates the rebellion of the cylons. Knowingly recognizing that some cylons would in fact need to be sacrificed in order to reach earth.

Laura Roslyn was also temporarily healed saved by the next evolution of the cylons (Hera). Whom she had ordered to be taken from her mother.

When Roslin arrives at the Ionian nebula, she nearly faints, possibly triggering another layer of programming.
In "He That Believeth In Me", Thrace hands Roslin her pistol and tells her to shoot if she believes that Thrace is a Cylon. 

Laura Roslyn was told by D'anna 3 that she's one of the final five.
From her cell,  Roslin eventually tells the Quorum of Twelve of her role in the Sacred Scrolls (more religious foreshadowing).

Roslin still holds a grudge against Baltar "The False Prophet" she tries to curtail Baltar's freedoms when his new-founded cult has violent confrontations with followers of other beliefs. Although she pleads to the Quorum to think of what Baltar did on New Caprica and what he might do with followers blindly devoted to him.

Laura Roslin also visits the Hybrid on the rebel basestar, but when it is awakened it purposely jumps the ship away as instructed by Laura Roslins programming, this triggers another level of pre-programing. 

Laura Rolin also like the other cylons needs a ultimate love (someone who will sacrifice themselves for her), her's will turn out to be Adama. Every time the basestar jumps in "The Hub", she has a vision of herself in an empty corridor on Galactica where she is joined by the spirit of her old friend Elosha, who takes her to sickbay to show her her own death. Elosha explains that Roslin has cut herself off from the ability to love people and shows her how Admiral Adama loves her.

Lastly, when she realizes that she had been discovered, she orders Karl Agathon to take Number Three to her once she is retrieved. From the ship, after the battle, the Three jokingly tells her that she is one of the Final Five, but refuses to tell anyone the truth until she is sure that she is safe. Number 3 has to submit to any question put before it by any of the final five.