Considering that audio demo tracks are going to be played many multiples of times, the most important thing for me is that I can stand listening to the music 50 times or whatever. My demo tracks are:
Wagner - Götterdämmerung, excerpts from Act 3, Solti "Ein Albe Führte Mich" (voices and orchestra)
Janacek - The Cunning Little Vixen, excerpts from Act 3, Sc.7 "A Vixen Running", Gregor on Supraphon (voices and orchestra)
Albéniz - Recuerdos de Viaje, Op. 71, No. 6 - Puerta Del Tierra, de Larrocha (piano tone)
Steely Dan - Negative Girl (complex mix, dynamics)
Keith Jarrett Trio - It's Easy to Remember (specially for the cymbals at the start)
Kim Burrell - I Come to You More then I Give (vocals + chorus)
I know all these extremely well, and still enjoy them.