I usually try out different tracks for the different qualities I'm interested in. For example, Machine Gun by Portishead is a good one for dynamics (just go easy with the volume).
Dear God Please Help Me by Morrissey is good for checking timbre.
You can do the same for sibilance, imagery, and bass depth.
Of course, well recorded albums such as Joni Mitchell's Travelogue will sound good on any system, but something like 60s albums by the Who, Kinks, Incredible String Band etc might actually give most systems a far tougher task of presenting a cohesive believable performance.
Piano is also useful for separating the wheat from the chaff.
In any case, it should always be music that you like and are familiar with. Most reviewers tend to stick with favourites, but these are usually their favourites, not necessarily yours.
With unfamiliar music, you're also basically in danger of getting overwhelmed with sensory overload.
