Ar Tonelico 2 has one of the most beautiful musics I’ve heard, not limited to games. Not too surprising, given how the entire franchise is basically about glorifying music to divine levels and brings top notch singers like Shikata Akiko and Shimotsuki Haruka to sing songs like METHOD_IMPLANTA and EXEC_SOL=FAGE/.. The BGM is also great, with tracks like Rustling Throb Cloche.

Other great OSTs that never leaves my phone:
-Einhander (Sample: Bloody Battle)
-Omega Boost (Sample: Orbital Flight)
-HellSinker. (Sample: Apostle of the Seed)

Finally, special mention to Space Invaders Extreme 2. I wouldn’t say it’s the best OST but it’s the most fun example of adaptive music I’ve seem (heard?). Edit: Also Silver Surfer from NES, given the miracle Tim Follins does to play this through its sound chip.