I think the main thing that strikes me about VII's OST over VIII and IX is that it has the strongest character themes of the three. You hear Aerith's theme and regardless of whether you like her or not you're probably humming along but Vivi who is a fantastic character has imo a pretty weak character theme that is easily forgettable because it doesn't really embody him like it should. I think this is largely why I like VII the best cause its character themes and slower pieces are pretty memorable whereas VIII and IX tended to go a bit overboard with re-using melodies throughout their OSTs so they ultimately blend in a bit too much for my taste. It also helps that VII re-uses its themes more often with only a few tracks being one shot whereas VIII and IX have quite a few more that only play once in the whole game so I never felt the OSTs had a strong base.
They are all great OSTs though, I just feel V-VII is Nobuo Uematsu's Magnum Opus.