I always find it hard to pick a favorite, but in this game's case that becomes a lot harder. All the music suit the moments they are used in so perfectly that what ends up sticking with me is how powerful the scene is or how much I like a moment of the VN. In that sense, Foolish Child is hard to beat, but I have some other close contenders.
A close second to me would "Greek Folk Song" by David Gilden. It has such a wistful and nostalgic feel to it that goes exceptionally well with so many scenes. I feel like I could listen to it forever without getting tired of it. With the Khenbish update I also got pretty happy to see they got some more use out of Signs (Part 1) cus iirc it had only been used on the ruthless routes up till this point.
Also on a song related note, I also find it strangely appropriate and a bit humorous that some songs related to The Valley and Argos come from the album "Shed Your Skin"