Not sure if this is the right thread to discuss BSG "guesses" but I don't think Apollo is a candidate because that would mean either Adama or his wife would have had to of been one of the final 5 so it would make it 6. Adama, Roslin, Baltar and obviously Starbuck are candidates. IMO Baltar and Starbuck are too obvious. Apollo could be if he was abducted and replaced but that is highly unlikely. Not sure if Tai was around durring the first cylon war, the first reference to Tai is when Adama got his first command so IMO the final 5 where around durring the first cylon war. Not sure if they kept the young final 5 cylons under wraps or where they were all this time cause they are to young to have been around durring Tia's time.
IMO Starbuck is the #1 option but it seems like the writers made that obvious to throw the fans off the scent. IMO it's probably going to be Roslin or Adama, since cylons can have kids Adama would work. I will throw my vote a Roslin, she makes sense and anybody else other than Adama, Baltar or Starbuck would be a total let down so it's really between those 4 and I'll go with the President by circumstance leader of the human race to be a non-human.