Music to test with:

I find that music that I'm really familiar with and really enjoy works well when auditioning systems. Stay away from music that you're unfamiliar with if you're auditioning a new piece.

I like Rickie Lee Jones 1st album, and Pirates. Willie Nelson's, Stardust album. Carmina Burana with the Czech Philharmonic. Suite for Flute and Jazz Piano, by Claude Bolling/Jean-Pierre Rampal. John Klemmer, Straight From The Heart, on Nautilus Direct Disc. For CD, I like Sade, Best of. Joni Mitchell, Hits. Sarah Brightman, La Luna.Madonna, Immaculate Collection. On SACD, I like Weather Report, Heavy Weather. The Bangles. Dvorak's New World Symphony, Bernstein.

The following are a reasonable cross-section of the various LPs that I often use to evaluatge a system:

Opus3 7900, Test Record 1: Depth of Image
(If I could only use one LP to evaluate a system, this would be it. Very naturally recorded acoustic instruments and voice with selections in a variety of genre from classical to jazz to folk. Listen to the small choir performing Isaac's Innsbruck to see how well the system can differentiate each voice.)

Argo ZNF 6, Holst/Savitri
(This recording has excellent reproduction of soundstaging and vocals. As soloists move on and off stage, the movement is clearly tracked on a system that reproduces soundstage information well.)

Audioquest 1001, Robert Lucas/Usin' Man Blues
(This Kavi Alexander recording tells me immediately whether a system can reproduce pace and rhythm - if this recording doesn't sound alive, there's a definite problem with the system. Unfortunately, I've listened to many expensive systems that fail this simple test.)

BIS LP 163/164, La Spagna
("Ancient music" recorded on original instruments in a church acoustic. Very interesting and distinctive textures and timbres that are challenging to reproduce / differentiate accurately. Side 3 is particularly telling.)

NorthStar DS0004, Arturo Delmoni/Songs My Mother Taught Me
(Wonderfuly natural reproduction of violin and piano.)

Klimo Open Window 002, Baroque Violin/Banchini
(Want another demonstration of the merits of simple miking and tube electronics - this recording will stress a systems ability to reproduce accurately the harmonic overtones of a violin and cello.)

Athena ALSS 10004, Stravinsky/Petrouchka
(Yes, Ansermet's strings can sound a little scrappy at times, but what a wonderful performance extremely well captured in this Decca recording re-issued by Athena. Side 2 band one for massed strings, then about the last 3/4 inch of side 2 for an entire orchestra going full tilt.)

Classics for Pleasure CFP 40339, Allegri/Miserere, Tallis Scholars
(If you ever need to convince someone that a system can reproduce the illusion of tremendous depth of field, let them listen to the antiphonal choirs captured in this recording. If you don't hear it on the system you're auditioning, look elsewhere.)

And then there is the Moeran, Lloyd, Arnold... But, this is enough for a start. I'll be interested to read other's favorites.

I think Buckingham is right on. That doesn't mean you can't become familiar with other recommendations. Fair warning, too much testing with your favorite music may make your favorite music less favorable.

Some favorites are:
Lyle Lovett: Joshua Judges especially tracks 3,4,6,8
Brian Bromberg: Wood, Awesome stand up bass and piano
Burmeister CD-III
Bobby Mcferrin: Beyond Words. Track 1 in particular

By the way I agree with Unsound. I can't tell you how many cd's I can't listen to anymore from over-use.