I feel redundant on these threads because to me, the music in a game is often the thing that pushes a great/fantastic game into incredible/perfection territory, and so my favorite games of all time (Xenoblade Chronicles and The Legend of Zelda: Ocarina of Time) also have my two favorite OSTs. Xenoblade Chronicles' OST as a whole is my absolute favorite if I have to pick just one though.
Favorite tracks from the Xenoblade OST:
Main Theme (on the title screen)
Confrontation With the Enemy
There's about 20 other tracks I could add to this list, but really the whole soundtrack is just brilliant. The 4 I listed are just some epic standouts.
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Definite: Kirby Star Allies (Switch), Mario Tennis Aces (Switch), Fire Emblem (Switch), Yoshi (Switch), Pokemon (Switch), Kingdom Hearts 3 (PS4), Monster Hunter World (PS4)
Considering: Fe (Switch), Donkey Kong Country Tropical Freeze (Switch), The World Ends With You (Switch), Ys VIII (Switch), Street Fighter V: Arcade Edition (PS4), Kingdom Hearts 2.8 Remix (PS4), The Last Guardian (PS4), Shadow of the Colossus HD (PS4), Anthem (PS4), Shenmue 3 (PS4), WiLD (PS4)