It's hard to pick a single favorite. The basic Seikilos Epitaph (in all its forms) is probably my overall favorite, while my favorite "upbeat" song is probably 2Virgule4. If I am looking at it as "best song/scene" combination, however, it's definitely Foolish Child. I struggle to put it into words, but I think that (for me) it just encompasses every sense of emotion that relates to romance; it is ethereal and mesmerizing, yet at the same time melancholy and bittersweet - which may seem contradictory, but as the story points out, the sort of love we consider romantic is only so powerful because we know it is finite and must eventually come to an end.