I don't know about "most beautiful song ever," but there is one little indie Japanese album I discovered on another forum that moved me to tears the first time I heard it. The whole album is just amazing. All it is, is a one girl and her acoustic guitar, with very echo-y, dreamy vocals.
Here, listen to this live sample:
As soon as I heard this I new I needed to buy her album: https://mayyyyye.bandcamp.com/album/
It's great to put on when I'm stressed and tired and maybe need some sleep, it always calms me down, like a magic spell it works every time.