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

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!

I have found over the years that a great test of a hi fi system is music that combines four things: full bodied vocals ( solo & harmonies), acoustic stringed instruments ( like acoustic guitar, banjo, mandolin, violin, stand up bass etc), amplified instruments like guitar, bass guitar , keyboards) & drums.  Not many systems can do this well at realistic volumes in a decent sized room especially when these sound components are all playing together. These very varied sounds & dynamic shadings often tend to get pushed together & loose their inner detail & nuance. 
 

A perfect example of this are the first two Jackson Browne albums which both sound very good on Qobuz if a good album copy is not available.