This is a tough one. Except for XII, I never cared for Sakimoto much at all, and while XII's OST is serviceable, I find many of the tunes forgettable or, in some cases, even vaguely grating and irritating.
XV has some great tracks - Gratia Mundi, Apocalypsis Noctis, Noctis' theme, Wanderlust, Cleigne battle theme, Somnus. Memorable tunes! But, while actually playing the game, much of my time was spent with silence or the not-so-great town and outpost themes. The situational stuff is cool, but you just don't hear it enough for it to sink-in in-game. My favorites in these games are usually tunes you hear over and over and that become ingrained in your memory of the experience. WoB theme from VI, VII, and X, for example. The way XV is structured, the best songs don't get revisited that much.
XIII has an awesome OST, extremely coherent across the whole game, dense while still catchy. Fang's theme and main battle theme are two of my favorites from the entire series. Stuff like the Oerba theme is haunting and super effective too. Still, the scope of the soundtrack is perhaps a bit limited. It's fairly monotone, which over 60+ hours leads to a sort of blending together.
Guess I'd give the nod to XIV. The Limsa theme is still burned into my mind, the traveling music in each area is fantastic and plays just enough to be memorable, but not enough to get annoying, the battle music is awesome too. This was my favorite FF OST since X. It doesn't quite match VI, VII, or X for me, but I'd probably rank it with IX.
XIV > XIII >> XV >>>>>>>>>>>> XII