Voted "Other", because (and this is going to be controversial) I'm going to say XIII-2. Yeah, you read that right. But if you put aside the rabid hate for a moment you can see that the OST for XIII-2 is stunning, and while about 8 or so tracks are dire for every dire track you have something like Chaotic Guardian, All or Nothing, and so on. It also took the music in a fresh and new direction and did it well (most of the time) while also including the sorts of tracks one expects from a FF game.
I therefore consider it not only a stunning addition to the franchise, but one of the finest OST's of the generation.
Also, excluding XIII and XIII-2 on the damn board for it's sequel is silly tc, especially when you included options almost nobody will pick suck as I, II, and III (because those would refer to the originals, not the remakes).
For its time?
It means for when it was released. Because no retro soundtrack when directly compared to something like, say, XIII's, can even hope to offer the same detail and level of polish to it's music.
Just like how VII by today's standards would be trash if it were a newly released game, but for it's time it was revolutionary and still receives a large amount of respect for that.