There are lots of “most beautiful songs ever” but “Sodade” by Cape Verdean singer Cesaria Evora is a strong contender for the best of the best. The song was written by Armando Zeferino Soares in the 1950s and released in 1992 by Evora on her Miss Perfumado LP. The song is about nostalgia and yearning and is sung in Cape Verdean Creole. Wikipedia has a nice entry on the song here.
Cesaria Evora was born in 1941 and passed away in 2011. My mother was a big fan (thanks to a CD I gave her for Christmas one year) and saw Evora perform in Atlanta in 2003 which she said was transcendent. Evora was nicknamed the “Barefoot Diva” for performing without shoes.