Anything Xenoblade, but especially 1 and X.
I got bored of Undertale quickly, but the OST is an all-time great.
Shovel Knight is really good.
Anything by David Wise, especially Donkey Kong Country 1 and 2.
Final Fantasy: Mystic Quest has the best music in the entire series, don't @ me.
The Grandia theme.
The Nier games have gorgeous music.