I would say the most beautiful song that I've ever heard is Guitarazzo from Astor Piazzolla's album Edicion Critica: Piazzolla Interpreta A Piazzolla.
For the most beautiful song I've heard that is possible to find on the internet might be one of these:
•Nightshift by Dave Brubeck
•Possession by Sun Ra
•Home at Last by Steely Dan
•Love and Hate by Jackie McClean
If you're a jazz fan you would probably know most of them, maybe not Nightshift or the Piazzolla, though.