One OST I have always been fond of is the one for Breath of Fire 4, a ps1 turn based game centered around, mostly, eastern mythology.
Playlist is here
My favorite songs from it are two of the battle themes and a character theme
The OST for Shin Megami Tensei: Strange Journey, a game about God, law, chaos, Lucifer and the fate of humanity.
My favorite songs are mostly atmospheric themes used in certain areas.
Area 1
Area 3
And specially the boss battle theme. It is slow and feels majestic and apocalyptic, like the boss fights in it.
And the OST for Space Funeral, a small RPGMaker game.
The battle theme is probably the catchiest song in the game