Maybe Queen's
Bohemian Rhapsody, because it's just full of different styles and shows excellent musicianship amongst all the musicians to come up with such a varied work.
In terms of impact on the world, John Lennon's
Imagine certainly helped bring some peace the world when it was released, so you can't discredit that.
But I'm going to choose a song I don't even like! George Gershwin's
I Got Rhythm from the musical
Porgy And Bess was the basis for hundreds - if not thousands - of jazz pieces. The chord sequence Gershwin uses (known in jazz circles as "rhythm changes"
proved easy to understand for the musician and yet still sounded good enough to sound impressive to the listener. Everyone learning jazz starts off playing "standards" - tunes that have become a standard for every jazz musician to play. And a lot of these standards are based around the chord changes to
I Got Rhythm. Even a lot of pop songs still used these changes. So that's why I'm choosing a song I don't even like - it kept jazz music alive throughout the 40s and 50s, even still into the 60s, and inspired thousands of songs.