Official BTS BSG Competition: The Fifth and Final., page 1

I choose President Laura Roslin I think it's the only option at this point actually.

Reasoning:

Not only is she the last 'person' we'd suspect, but she's just so adamant about finding Earth. She's willing to tear the fleet apart if it means meeting that ultimate goal. This is evidenced by her arrest, subsequent escape and the separation of the fleet in the season 2 premier Scattered. In this episode President Roslin and Capt. Lee Adama escape their imprisonment and split the fleet into separate parts. Those loyal to the President, and those loyal to Galactica.

Now it may be argued that Commander Adama played as much a role in that separation as Roslin did, but I'd mention that not only was Cmd. Adama shot by a known Cylon (Boomer), but he also has two known sons. Excluding the 'Flaming Arc' saga from the BSG comic book, Cylons don't have families.

I'd also point out that Roslin's battle with cancer put her directly in the role of 'Dying Leader' as prophesized in the sacred scrolls.

"And the lords anointed a leader to guide the Caravan of the Heavens to their new homeland."

It's known that the Cylons know much about the mythology of the 12 Colonies, and have no problem using it for their own needs. If it was planned by the Cylon God, or whatever powers rule them to take an active role in humanity finding Earth, then it can be presumed that they would also place key figures in their ranks, including the absolute spiritual and political leader.

Another thing I'll point out about Roslin's cancer is that it is recurring. Although magically 'cured' by the blood of Hera Agathon the half Cylon, half human hybrid daughter of Helo and Athena. This was meant to completely cure her of her cancer, but it would also remove her from the role of the dying leader. It stands to reason that the Cylons may have pre-programmed her body to regenerate the cancer in the off chance that it went into remission, or was cured by the human doctors.

My final point is the Opera House. President Roslin, Athena and Number Six all dream the same dream of the Opera House. They dream it as if they were sharing the dream, each with a different role, yet all aware of each other's presence. This seems odd to me that a human (even one with such a prominent role as Roslin) would share a psychic bond with a Cylon. The only other time we've seen anything remotely close to this was when the first 4 of the final 5 heard the same music playing, drawing them closer to each other. That's not to say that it isn't something of a higher power at work, but I see it more evidence of Roslin's true Cylon self.

But then again, maybe that's what they want me to think........