What music do you use to assess your system?

I try to take something that has very complex piano parts - Yelenna Eckemoff's Cold Sun comes to mind - because in my experience a system that reproduces a piano well - with all of its subtlety-to-bombast and everything in between - can do just about anything I'm going to want it to do. Very difficult instrument to reproduce truly well.

Here's mine;
Jamie Cullum -twentysomething CD/SACD (2004).
Miles Davis- Kind of Blue CD/SACD (1986, 1999).
Paul Simon -negotiations & love songs (1988).

Keep this train a rollin'

I can appreciate the fact that to evaluate a systems potential, the best recordings available would be used.
But,I actually also try to listen to the music I play and enjoy the most. Say, the best audio version recordings of CCR.

I like to listen to everything, all genres regardless of recording quality.

So I assess by randomly queuing up tracks to get a random sample of how things sounds overall.

Every recording is recorded differently, so I listen to make sure I can hear what is going on in each clearly, for better or for worse, and take note of the variations and unique aspects of each recording

Then I just listen to whatever I want to. If I hear a problem anywhere, I take note and address it, if needed.

Another benefit of using a random sample is that one is better able to determine when a particular sonic deficiency is common across multiple recordings (most likely a system problem) or not. If not, then sonic deficiencies can be attributed properly to the recording rather than the playback system

These days a few of my favs are Shonen Knife's Let's Knife, Dylan's Modern Times, Led Zeppelin's Mothership Disc 2. Just for testing purposes, officer.