Okay the first one that everyone can usually think of is Harry and Draco. This is especially apparent in the 6th book. This is the book that Draco is chosen to kill Dumbledore--so he is a chosen one. And this is the book after the prophecy that reveals Harry Potter as the chosen one which becomes a major issue in this book. But of course Draco is trying to prevent the good side from moving forward; while Harry is trying to prevent the evil side from moving forward.
But now I have some great examples of foils :)