To Zanarkand is the highest on Spotify by quite a number at 11 million plays
highest from VIII? Balamb Garden (3 million compared to 1.8 for Liberi Fatali)
highest from VII? the Battle Music actually squeaks past the boss music and One Winged Angel, all three have about 4.6-4.7 million plays
VI's is the boss theme at 2.2 million plays, IX's is Melodies of Life at 2.7 million
Edit: Actually XIII's main theme and Promise By the Light are at 5.4 million and 5.2 million respectively, Somnus from XV... at 9.2 million (Who are these people listening to it haha)
Warning: I'm literally VeryInsane.