I think the most powerful out of those two would be percy jackson because he has control over water and if he destroyed Kronos, he can probably fight harry potter. I apologize to any of the Harry potter fans if I am offending you. I am a serious fan of both, so it was really hard for me to choose.
Harry Potter would definitely win. He has way more things that he can do against Percy Jackson. There are 73 spells that he is capable of doing and is not limited, such Percy is with water. Harry can deflect, battle, and help using his wand and that gives an advantage.(Though I love both too, I stand to my point.)