Not best song , better to use your ""ears""
I don't know if my favorite music is appropriate for another person to test speaker performance with since what we like is all personal taste.
My approach is to bring my favorite music (both poorly recorded and well recorded).
But I have 3 albums I use for testing Dire Straits Brothers in Arms, Bella Flec Flight of the Cosmic Hippo, and Jeff Beck Wired.
If a speaker system can satisfy me with these albums I am good to go.
Some great suggestions here, thanks.

However, with one or two exceptions, there's very little here in the way of excellently recorded orchestral music, which I think presents a unique test for loudspeakers.  Mercury Living Presence has some good stuff but the application of modern recording techniques has raised the bar substantially.  

Any recommendations in this genre would be appreciated.
I use different kinds of music to explore various performance characteristics of speakers, ie:

  • Well-recorded acoustic jazz to explore timbre, detail/speed & soundstage
  • Well-recorded large scale classical and choral to explore time, detail/speed, soundstage, and dynamics
  • Well-recorded blues, Afro-pop, reggae, electronic/chill to explore overall tonality, bass depth/impact & dynamics

One of my favorite torture-test tracks in the 3rd category is Donald Fagen's "Morph the Cat" (title track of that 2008 recording). The opening note is a huge transient built up of countless bass instruments & other things in the studio. Few other cuts show just how large & loud a speaker can get (in a hurry).
For overall 1 disc to have (if you can find it) I choose Opus 1 (1st) test CD .. amazing breadth of music and well recorded .. Tiden barogue (unsure of name/spelling cut) I have found glaring distinctions from 1 system to another .. and while its not my "relax a bit CD" it allows me to quickly size up a systems strength for placement, depth, stage width & especially detail.
An all out dynamics and bass challenge would be Gordon Goodwin's Big Phat Band "Life in a bubble" (HDTracks & others) if your not hooked by the 1st track .. you need to turn it up louder.