Well, there's all kinds of stuff out there... Any specific genres you're looking for? I'll offer some random stuff here:
Bossacucanova: Essa Moca ta Diferente -- album is Uma Batida Diferente
Pink Floyd: Dark Side of the Moon -- whole album
Aaron Copland: Fanfare for the Common Man -- not sure on album
Air: The Vagabond -- album is 10,000 Hz Legend
Paul Simon: Diamonds on the Soles of her Shoes -- album is Graceland
I'm actually planning on doing a segment on how to put together a listening test disc in one of my next 2 or 3 episodes. Paul Simon "My Little Town" not as a show off piece, but to test difficult material. The recording is a little muddy, but the instrumentation is still really dense. If a set of speakers can make that track sound open, then you've really got something.
If you are looking for recordings that on a right a properly designed [low resonance, wide dispersion omnipolar] and placed [within an appropriately acoustically treated environment] system sound like real life Mapleshade Records has a large selection of titles. The largest problem with their recordings is that they insist on using analog recording methods which means there is audible tape hiss. Besides that one issue these are some of the most realistic recordings I have experienced both in a tonal and timbrel sense along side imaging, listener location etc...
Ethan Frome is one specific opera that is a great example of what I am talking about.
I would suggest if this is to impress other people with your system, make them realize how music can sound, or sell them on what you've got, it would be best to use material that is very familiar to them. Perhaps very popular music that everyone knows, or have them bring material that they are fond of. If they've never heard their music on a really good set-up, it can be quite enlightening to them when they hear details, and other things that they've never noticed before. Of course good demo material may make them think WOW that sounds good, but when it's something they've heard a thousand times and they know the track inside and out and find themselves thinking "I've never heard this sound like that!", it's even better.
Trouble with that is that most list-pop these days only sound good on a clock radio...
True...I often forget that there's no accounting for taste. Although I'm sort of assuming that people who are truly interested in good sound and are passionate about their music will not be bringing that stuff for demo material. Not all pop is bad though even if it is heavily compressed and has very basic sound palette used. Example MJ's Thriller album. Everyone has heard this album a ton of times on a bunch of different set-ups and you think you know the track, but when you hear it on a really good system it can surprise you.
Personally I like the track Stones from the Sky from the band Neurosis for evaluating speakers.