You people keep on ignoring the facts, but when it's one of 12, I'm coming back here to laugh at you.
The whole "I can't believe it's you" even furthers MY belief. He's saying it because, out of the 12, he can't believe that it's this one particular person.
But no...you're right. It's Peter. Yeah...and I'm Tim Kring.
There are no facts yet, unless there is some spoiler out there that I haven't read. If there is, don't tell me what, I'm not reading spoilers.
I don't recall exactly how he phrased his disbelief, but I don't think it indicated if it was part of the 12. And it seems like the 12 don't really trust or like each other, so why the disbelief? Plus, if it was one of the 12, that would seem almost too cliche.
I don't think it's Sylar because the build doesn't fit, and it would be corny for Sylar to be the shadowy mysterious figure again.
My prediction is that it's one of the Petrelli brothers. The hooded guy fits both of their builds. And the "I can't beleive it would be you" would make sense because he couldn't beleive that they would kill their own mother. Maybe as more people die, it turns out to be both, yet neither of them know the other is also killing. Some clues:
Nathan - Could easily fly away after knocking Mr. Nakamura off the building. The scar he saw in the mirror, maybe not a physical scar but a reflection of how he feels in his soul. He also knew of his mother's plot to sacrafice Peter to better the world. Revenge is a good motivator.
Pete - Could easily fly away or teleport after knocking Mr. Nakamura off the building. Also, after Nakamura was knocked off the building, Peter showed a new ability to shoot electric bolts from his hands. They never mentioned what Nakamura's power was, so maybe that was his power that Pete absorbed.
People have stated that maybe Peter absorbed Nikki's split personality. I just have a few comments about that. I personally never believed that Nikki's split personality was part of her power, but something more personal shared between the sisters, so it isn't something that can be absorbed. Also, even though Nikki wasn't aware of what Jessica did, she still knew who she was. Peter had flat out amnesia. Maybe the Haitan had something to do with that. After he showed up in New York to surprise Claire, he kind of dissapeared. We don't know what his true intensions are.