For me, my top choice, though it is so hard to choose just one- is 'Strange Fruit' written by Abel Meeropol and sung by the very talented Billie Holiday.
This song was the first protest/political song-
It took a lot of bravery and a bit of a rebel spirit to take it on.....
It was made all the more powerful being sung by Billie b/c of her race, sex and her passion behind it.
At that time race and sex were an issue, I believe that this song helped to open many doors for so many musicians who came after Billie.......It changed the history of music.
I love the song, when sung by others as well- but Billie owned it.
So, that's my top choice.