Thalia: Destined to be fifteen forever, as a Huntress.
Nico: At the end of book three, Percy says, "I don't want this kid to go through anymore hard times." Or something like that. Percy won't let him be the child of the prophecy.
Percy: DING DING DING! We have a winner!

I also think it's Percy, but Percy doesn't really have control over who the person the prophecy was made. I don't think it's Nico because he won't be old enough.
But there's also a good chance that it could be Thalia. All she has to do is break the oath of not loving a man, and then she'll turn sixteen in a few hours.

I don't no it could be Percy or thaila she just has to brake the oath I mean it not going to be nico well maybe it could be nico if Percy dies and thaila brakes the oath and there would be a whole new book but why would rick change the main charater I mean come on wait wat if thaila does brake the path on percys b-day then it wouldn't be just one persons decition it's two kewp

Percy or Thalia. Most probably Percy, though, just because he's the main character in the book ;)

DUH!

percy is the child of the prophecy.
luke is the hero.
Annabeth gets to be the architect she dreamed of.
grover is apart of the council of cloven elders.
they all live happily ever after THE END..
until then...