Well, Zoro from One Piece is an incredibly skilled swordsman. He has trained hard and has his own unique fighting style. But in Percy Jackson, there are characters like Nico who can control the dead. If Nico were to summon an army of the dead in a battle, it would be a very different scenario. Also, Percy can create powerful water - based attacks. If he were to use a large body of water against the One Piece characters, it could be very difficult for them to counter, especially those who are Devil Fruit users as they are weakened by water.
I think Whitebeard from One Piece would be a top contender. His quake - quake fruit gives him the ability to cause massive destruction. But Percy has his own set of skills. He can heal himself to an extent with water and has a lot of combat training. On the Percy Jackson side, Annabeth is also very smart and could come up with strategies. In a battle, she might be able to find weaknesses in the One Piece characters. But in One Piece, characters like Doflamingo with his string - string fruit can be very tricky opponents too.