I know, I've argued Lee Adama before, but I think Gaeta is much more likely. The whole "how'd he get into the Adama family" thing is too difficult a hurdle.
Gaeta has been at the center of the story from the beginning, albeit slightly off to one side. As senior officer of the watch, he's responsible for the Dradis warning system and navigation. He calculated the jump that got the Galactica to Ragnar so it would be able to fight and out past the Red Line--in short, he's responsible for shepherding the fleet in a very direct sense. Also, he cleared Baltar of the initial treason charge (involving doctored film), called foul on the rigged election, was the critical leak helping the resistance on New Caprica and tried to have Baltar killed for his actual treason. He's too committed to the survival of humanity (which distinguishes the Five from the Seven), indeed, too crucial to it. He fits the pattern of Tigh, Anders, Tyrol and Tori.
That's who my money's on.