Final Fantasy VIII. No other FF had a set of tracks that etched itself into my memory quite like that one. I'm a massive sucker for organs and beautifully haunting pieces and this game has them in spades. And the hype music, man. The Stage Is Set, The Mission, Movin', Only a Plank Between One and Perdition, the list goes on. Stellar stuff.
It is underrated, but we're talking soundtracks, and for me, a piece needs to memorable in both being something I can hum along with and something that evokes the experience of the game. This usually works well in games like IV and VII because the music is used to add emotional depth to a scene. When you hear it, it should take you back to that opera scene or that fallen city in the rain. XII's OST is great for being background noise but doesn't lend itself well to infusing itself to the scenes itself. There's a disconnect.
IMO this is something XII actually did very well with area/dungeon themes. Every single one of them (besides Tchita Uplands, screw them) feels very distinct from all others both in aesthetic and music. I can think of any area in the game and the BGM immediately comes to mind. Even all the different mines and woods have a totally different feel to them, though a possible exception admittely might be the western deserts, at least pre-IZJS. Still, this is something XII is better at than any other game in the series, especially the SNES era where, albeit justified by hardware limitations, every similarly themed area felt the same.
I'll agree that the OST doesn't really capture particular story elements, but no other OST brings back the feeling of actually sitting down, playing the game and exploring the world that well.