I'm setting out to define what is the worlds greatest acoustic version of a song.

Largely looking at songs that were originally non-acoustic song and then the acoustic version has become the definitive version of that song.

Definitive can be a little bit of a subjective measure. Example. I believe Foo Fighter's own acoustic cover of Everlong is personally definitive but not for the rest of the world based on spotify plays. 1.8 billion compared with just 295 million for the acoustic version.