Music that you like is always a good thing to show off with. That or something the person you are showing off to likes. Doesn't matter how accurate it sounds, if I hate what is being played, I probably won't be impressed. If someone were to throw in some hardcore rap to demo something to me I would walk away. I wouldn't even give it a chance cause that stuff annoys me so much.
If you want to have a way handy to show off the system, burn a CD with a huge variety of well recorded material. That way you have something that will appeal to whoever is listening to the system. Obviously use a lossless format.
There have been a number of excellent recording already mentioned but one of the classics that hasn't is Jennifer Warnes-Famous Blue Raincoat