To test imaging, it's hard to beat any of Diana Krall's CDs, the recording quality is amazing on them. Although these discs can exploit problems if you are familiar with them. I would not recommend this as an "impress your friends disc".
To test the transient ability of subwoofers, there are a couple of standby's. Metallica's "...And Justice For All" with it's double bass drum can really give subs a workout. Another is Telarcs 1812 Overture and the canon blasts. Organ music can be very dynamic as well.
To impress the friends, you'll probably want to use some modern hip hop.
One disc that surprisingly sounds good on almost every system is....i'm embarrased to admit this....Madonna's Immaculate Collection.
