What is the most challenging music to play on a stereo?

It's just like movies, you'll never learn anything about movies by watching the best movies ever made you'll learn a lot about movies by watching bad ones. Listen to poorly recorded music distortion that couldn't be fixed see if the speaker reproduces distortion well  (that's hard). Square waves, industrial "music" that is blown out, is a great test.

I agree/forgot Piano,

1st a great piano RECORDING (hard to get right), then how does it sound?

Solo Piano

String Quartet

Female Voice.

Many, if not most systems have problems with sibilance. Female voice and violins commonly bring this out. Only the great systems can cruise through these without biting. Great does not mean expensive. I just heard a system built around Harbeth P3s that was wonderful. Piano due to it's complexity and sustain.

Zamfir

A good RECORDING is the best bet. Since all recordings are "processed" in some way by the time we get them, I would seek out something by Mayorga or other direct-to-disc recordings since they are the least processed. Loving to hear Baez do Diamods and Rust is nice, but how much "stuff" was done to the recording to get that voice so crystal clear and those dynamics so balanced?

As I was told all my life when trying to decide something, CONSIDER THE SOURCE...

To each their own, but I would start with a quality recording and go from there.

Cheers!