Imo the greatest song is different from the greatest performance of a song. E.g. John Denver wrote amazing songs but didn't have the greatest voice. But there are songs that aren't as amazing that suddenly sound like the greatest of all time when the right person arranges or sings them.
But then there are songs with specific arrangements that are carefully crafted works of art, like Bohemian Rhapsody.
Also, there are timeless folksongs, lullabies, holiday songs, and songs from musicals to think about too. And should the greatest song be something that can be enjoyed by all? I love Disturbed, but I don't play it around my 3-yr-old.
I will not be able to decide this today.