Much like Sean I tend to listen to quite a few different pieces of music when I'm assessing a prospective purchase or trying to ascertain if I have set my system up properly after a good faff with my CD of sine waves! I always think a good system can do Simon & Garfunkel, Slayer, Tchaikovsky, Pendulum, Bowie, Lady Gaga etc. etc. ad infinitum... Everything basically, I have very eclectic tastes, but even if I didn't I would tend to assume something was wrong if a system could say do classical, but not rock - accurate is accurate!
Tracy Chapman, Fast Car is my usual first track, partly for nostalgic reasons (first song I heard through a "proper hi-fi" vinyl fronted system), but also because between the vocals and the bass it quite quickly shows up the systems character and draws attention to any issues. Monotonal bass or crossover/driver integration mucking up vocals - if I suspect the latter I usually skip on to Behind The Wall - the vocal only track makes it clear!
Slayer, Raining Blood is quite often the second as again it can very quickly highlight deficiencies in a system, especially jitter/speed instability, lack of power, poor dynamics. Played at a fairly high volume it is a torture track of sorts - the most common criticism of that sort of music is that it is just noise, and on a poor system it is - on a very poor system you can't even separate the guitars and the drums! On a good system - well it might not be to your tastes and will probably offend your sensibilities if you're naive enough to think they take anything they sing about seriously, but I don't think anyone who has tried to or does play guitar, bass or drums in a band could fail to be impressed with the speed and complexity of it - more importantly the better a system is, the easier it is to discern which instruments are making which noises!
After that, bit of classical, bit of less aggressive rock, bit of compressed pop... - depends what I feel like really, I sort of proceed randomly on a "wonder what that sounds like" basis. Imogen Heap, Hide and Seek is a recent addition to my favourite demo songs too, I know that reverb is digital - you can tell (or should be able to), but whether it seems to wrap around you or is stuck between the speakers tells you something!
