I don't think there's any "best" soundtrack since my enjoyment of them depends on my mood and other factors, but I'll just name the one I most often jump back to

Ar Tonelico 2, not only is its regular music good, the real greatness of its soundtrack lies with the vocal songs (which it has a lot of, being a game about a world where songs can literally be magic). Although I imagine the impact they have on me is enhanced by knowing the story context for each of them.

Example: