If by beautiful you mean something that has a transcendant quality and great emotional impact, then that is a very personal choice.
For me, Jez Lowe's "The Last of the Widows" is the one; he waited to write a song about Easington's mine disaster in the 1950s until the last of the widows had passed away. It is a beautiful melody and deeply moving lyrics; just writing about it brings tears to my eyes!