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

The Cd's I audition with are:

1. Classical - 1812 Overture - Sir George Solti - Chicago Symphony Orchestra - London Decca - 417-400-2 [Full orchestral range, plus bells and canon booms]

2.  Joan Baez Greatest hits - in particular "Diamonds and Rust")  - A & M Records - CD - 0510

3.  Sarah McLachlan - Wintersong - Arista records - 82876 - 81504 - 2

Mumblecore or solo zither. Or maybe anything you're familiar with? It doesn't matter what sort of music you play to test a rig other than you know what the hell it's supposed to sound like, and can then judge if the rig is playing it accurately. 

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.