For me it would have to be classical music, although I listen to jazz and rock as well, as there is nothing, at least in my mind, as difficult to make sound natural as music in a concert hall.
1) Gershwin's An American in Paris which is side 2 on a specific vinyl recording (from the TAS best vinyl recordings list), an RCA Living Stereo recording with Arthur Fiedler and the Boston Pops. Easy to find streaming as well as on cd. It is a virtual obstacle course for a system. The shrill woodwinds, full string section playing extremely high notes together and sound effects will really test a system. If you're hesitant about yours passing, my suggestion is don't try it. It is a great recording and totally resolvable but not by every system.
2) My number 2 choice is any number of solo piano pieces because what I am looking for is it actually sounding like a grand piano being present in the listening room. Liszt Sonata in B minor is a good one.
If you pass these then human voice, jazz, and rock are a piece of cake in my mind. Your results may vary however.