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

@roxy54    You nailed it.  Massed violins.  Far too often they sound like mush, even on costly systems.  Especially on digital.  It seems the digits have trouble resolving multiple HF sounds that are all playing the same tune.  If you can hear individual instruments on your system then it's tops.

After that, solo piano.  Does it sound like a piano?  Too often, not entirely.  However, poor piano sound is often down to the recording.  Miking a piano is very difficult.  Too many close mike.  That is not what is required.

Those who said 'vocals' are incorrect.  The human voice has tones in the middle, easy part of the range.  Most systems reproduce them realistically.

+1 Kota

Iron maiden.  Rhyme of the Ancient Mariner, at full volume pushing to and maybe right up to over over 110db..

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.