Favorite song to demo or test drive speakers?

Opalesque by Ralph Towner and Gary Peacock

Immortal Bach by Nystedt

+1 Jan Hammer/Jeff beck song. Great choice. 
Walk on the Wild Side, Lou Reed. (Background singers!)

Poinciana by Keith Jarrett Trio, Live in Paris (drums first, then the most beautiful sounding piano—great for comparing DACs)

Follow Me Home, Dire Straits (soundstage test for ocean waves, then bongos, drums, then bass and various guitars...and then vocals). This track will mesmerize you on a good system. 

+1 Jan Hammer/Jeff beck song. Great choice. 
Walk on the Wild Side, Lou Reed. (Background singers!)

Poinciana by Keith Jarrett Trio, Live in Paris (drums first, then the most beautiful sounding piano—great for comparing DACs)

Follow Me Home, Dire Straits (soundstage test for ocean waves, then bongos, drums, then bass and various guitars...and then vocals). This track will mesmerize you on a good system. 

There are a lot of great songs here -- I just threw a couple up without much bothering to say *why* I use them. Without saying why, it just seems like I'm telling you about my favorite songs, and that wasn't the OP's gist.

So, here's a good one, with a "why":
https://www.amazon.com/Recat/dp/B005J57SK6
"Recat" is on Ricardo Villalobos and Max Loderbauer's album, Re: ECM.
It is a weird electronic track which begins with hypnotic drums and little electronic pulses, some thrown all the way behind your right and left ears; soon, a melody begins to emerge, almost as if it was underwater, far away — it sounds like a merry-go-around melody, looping, getting closer.
The whole thing is rhythmic, bass-y, with a very interesting sound stage that pops in front and around you, like fireworks.
I tried it on different speakers and amps and while it's always fun to listen to, when the gear is good, you'll forget yourself.