An alien intelligence has descended and demands that you produce the single piece of music that you most love, to save the human race. What do you choose?
Maybe it's better to pick progressive rock from the 60s or 70s, the times we thought about space with intrigue and imagined the whole other world would be built by now. Maybe Interstellar Overdrive by Pink Floyd would be a fit.
Yet, I'd go with Assembly of Light - Transition. Besides my preferences, the eclectic nature of this song (thing as ancient as chorus vs pretty modern drone of morbidly overtuned guitar) may demonstrate, how we are too a complex of seemingly incompatible things. Emotionally, it feels like a relief after overcoming a struggle, a thing that I find is pretty universal to all beings. And it also doesn't have lyrics, so there won't be any misunderstanding.
There's a high chance I get executed as a traitor to humanity, but I couldn't live with myself if I skipped the chance to send this to aliens as part of first contact:
https://m.youtube.com/watch?v=dQw4w9WgXcQ
If it's to persuade the alien of the value of humanity, I'd probably go with The Lark Ascending by Vaughn Williams. It's beautiful, and also 15 minutes long, giving us some time to think of a plan in case they don't like it.
If it's just, what's your favourite song, it'd be So. Central Rain by R.E.M.
Either full album from Little Mazarn. Io and Texas River Song have a sincerity that could spare us. They would show the gentleness, nuance, and sophistication of our species. And Marfa Lights might be about them! If I had to pick a single song, it's definitely Peace Like a River, the first track on Io, about living with peace and understanding in the face of struggle and loss. I'm going hard on the wager that we don't want to come off as erratic or dangerous. We want to be portrayed as a safe potential new member of interplanetary society.