Referrence music....

Among many recordings, I like any of the piano trios performed by the Beaux Arts Trio. Just nice and clean violin, piano, and cello (high, mids, and mid bass) on one recording with many solo sections for each instrument.

I find that usually if the piano sounds right, everything else will fall into place. So some solo piano is a good secondary.

I agree female vocals are also on the list. Then some Tchaikovsky and Shostakovich symphonies to make sure a full orchestra does not become mud.

To top things off, Dave Brubeck's "Time Out"....

A CD that's really useful is Holly Cole's "Temptation." I will do a great job of testing how your system handles female vocals, piano, bass (acoustic) and silence.

Dire Straits, On every Street, song You and Your Friend
Excellent vocals, imaging and best example of a decay of a note on a digital format.

The "Depth of Image" LP from Opus 3 is also a favorite of mine. As Newbee comments, all of the cuts are of voice or acoustic instruments, it has a wide variety and combination of instruments, all the selections are well recorded, and they all come from different recording sessions in different acoustic environments. If you're using only digital sources, Opus 3 has compiled many of these cuts plus some different ones on their Test CD 4. WELL WORTH having on hand.

Whatever you choose to do, though, don't get trapped into listening just to a small ensemble (solo voice and a limited number of accompanying instruments, for example). You run the risk of assembling a system that will sound great on that one recording, but not on other things. And, using more complex music will tell you more about how well the system can articulate multiple instrumental lines without becoming congested. I've listened to many systems that have sounded great with a small jazz ensemble or a singer/song writer, but simply fell apart or became incredibly congested on more complex jazz or classical music.

As Sugarbrie suggests, add some well recorded large orchestral to your critical listening - it will tell you a lot very quickly once you learn to listen into the fabric of the music and not to the hyperbolic bombast of that latest great drum thwack. If I were to choose just one orchestral recording, it would be the Mercury recording of Stravinksy's Firebird (Dorati/LSO), particularly the second half. And, if I were going to evaluate a system with just one recording, this one recording would tell me virtually everything I needed to know about that system relative to my listening priorities.

I play one of my favorite songs, partly because I know it so well, partly because it contains a suitable gambit of sound quality "tells" and partly because I'm just excited to hear one of my favorite songs, but better. And then I play other songs which have different characteristics.