Nico definitely. He can summon the dead. If Percy brought an ocean, Nico could make all the fish-skeletons come to get him, and if Thalia summoned wind, he could summon a special sea-dwelling pterosaur zombie and ride it through the storm XD That's not to say I don't like Thalia and Percy (though after reading The Mark Of Athena I am seriously sympathizing with Nico), I just think Nico's powers would be stronger, as Thalia's and Percy's only have power over certain elements, whilst Nico has power over anything that ever died.

My guess is that Thalia might be stronger than Percy sine she commands Thunder and Water is weaker against that. But I still chose Percy. XD :) :p

Nico because he is the frickin son of Hades!