I think this might be an impossible task. I don’t think you’d get enough majority consensus worldwide for one song to come out on top. Suggestions on here are, unsurprisingly, English language songs with western tonal values however, there are billions of non English speakers in the world who would not consider these songs greater than a song from their own culture.

Even within a western sphere, there isn’t consensus. I performed a cursory search of the best French songs of all time and non of the songs commented here came up.

It is also dependent on what you want from ‘the best’ song. Is it more important for the song to sound uplifting or do the lyrics need to resonate with the listener.

Perhaps a better way to look at this is if an intelligent alien came to Earth and asked for one example of what music means to us, what one song would you present?

Even then, I still don’t think we could do it. I for one don’t know enough about other cultures music to even begin to approach this and I suspect I’m not alone.

Just for fun though I’ll put my two pence in and say: Ne Me Quitte Pas by Jacques Brel.

https://open.spotify.com/track/3IGb1Cn9P0tw4ksAHWneIX?si=A5722cYPQyS88OMyCSYNJg&context=spotify%3Asearch%3Ane%2Bme%2Bquitte%2Bpas