Is Stannis, not Jaime (as most of us were hoping).
Cersei asks Maggy if she will wed the prince (Rhaegar), to which Maggy responds that she will instead wed the King. Cersei then goes on to ask if she will have children, to which Maggy responds that the King (Robert) will have 16 and Cersei will have 3. Then Maggy says this:
"And when your tears have drowned you, the valonqar shall wrap his hands about your pale white throat and choke the life from you."
Well, Tyrion is halfway across the world where the story left off, so I'm ruling him out for now. I'll reconsider Tyrion as the Valonqar when it looks like he's heading back to Westeros. Plus Cersei was expecting Tyrion to try to kill her because of this prophecy and, while it's still possible, I'd be more amused if GRRM shat on Cersei's expectations.
Now Jaime...Jaime only has one hand. He can't wrap his hands about her neck for this reason. If you want to be technical and say "Durr, he has two hands, but one's golden", well...how do you "wrap" a hand made of gold around someone's throat? Hmm?
When Maggy said this, she'd just finished talking about the king (who we know is Robert). Right after mentioning Robert, she mentions the Valonqar; the little brother. Is she referring to Robert's little brother?