Long time reader, first time poster. I've read a lot of posts, not just here, but all over the internet on speculation on who the final cylon is. Most of these seem to come from a gut instinct, some personal feeling, and sometimes, blatant speculation. As with any mystery, the only way to solve it is to deduce the answer. Let's analyze what we know about the final cylon.
- He/She is not in The Last Supper picture
- He/She is not in the fleet at the time of D'anna's return (4x10 - Revelations)
- And the fifth, still in shadow will claw towards the light, hungering for redemption that will come only in the howl of terrible suffering. (Razor, Hybrid)
Who fits all of these? Kat.
Starbuck found out that Kat wasn't who she said she was, and threatened to expose her. Kat's response? Successfully committing suicide in a redeeming manner. Oh yeah, also, in a large cluster of light. (3x10 - The Passage)
This will answer the question of whether or not the Final Five can resurrect.
RDM has also said the final cylon will not be some day-player from season one. Or a guest star.
If you want to include Leoben's line in 1x08, "Adama is a cylon", well.. RDM explicitly stated that the moment between Kat and Adama was meant to imply his love for her as a daughter. Sloppy explanation, I know.
Someone on io9 questioned about the radiation not having an affect on cylons. Well, in the very same episode, Sharon is clearly affected by the light cluster, Doc Cottle suggests although she's stronger, she's also in mortal danger. Which explains why Sharon wasn't in any of the Outband jumps accompanying fleet ships.
Some argue that Ron Moore said the Fifth is not in The Last Supper. Well, D'Anna said the Fifth is not in the fleet and all of the human characters in that picture ARE in the fleet. Perhaps that's why he didn't want to let it slip, because it was a reveal for the finale.
But rest assured, the final cylon, is Kat.