I Can't Dance is on my list for the vocal alone. Very dry and recorded very warmly. Shows off the lower-mids.

Cherry Twist has some deep-deep bass hits that'll work out a subwoofer.

The thing is, there's probably not a "one size fits all" demo song. Those two songs only show off certain aspects of the sound system, you also need demo tracks for stereo separation and imaging, high frequencies, etc.