I'm torn; part of me thinks it should be one of the undisputed gems of the Western Classical [sic] Music repertoire, e.g. Samuel Barber's Adagio For Strings, or the Prelude from Richard Wagner's Tristan und Isolde, or one of Ludwig van Beethoven's symphonies, or one of Elliott Carter's string quartets, or one (all?) of Igor Stravinsky's ballets, or...
But the other part of me thinks something raw, naked, stripped of all learned craft, and performed with a complete lack of pretense, would be far better suited at conveying to a total neophyte what it is that music is capable of ...and so that part of me wants to suggest some anonymous farmer's wife humming "Oh Shenandoah" as she hangs the laundry up to dry, or some equally anonymous muezzin reciting a call to prayer from atop a distant minaret...
Or maybe just play them Exile On Main Street