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

I'd use the type of music that I listen to, which is rock. I don't care about a system's ability to play Diana Krall etc etc etc 

badly recorded music over classical. Classical recordings tend to be well done and so they can sound decent and enjoyable on even cheaper systems. Then there is sound chaos that sounds sweet on a car radio but need a lot of tweaking on a serious home hifi setup to avoid headache and ear-bleed. 

@edcyn

Vocalists. Any genre. Do the male voices suffer from a buzzy midrange distortion? Or do they sound rich, clear and authentically human? Are the sibilants spitty or hashy?

+1

Vocals are the most challenging. We hear human voices every day for a reference. Do they sound natural in your system? (Allowing for a slight touch of reverb.)

For something different, visit a piano dealer. While someone playing, listen from 5 ft away. 15 ft away. 25 ft away. It will be educational.

Getting both vocals and a grand piano right would the ultimate success.

@larsman 

FYI, there is a global listening party for Metallica's new album in movie theaters being played in Dolby Atmos on Thursday April 13th, don't miss it:

https://metallica.film

To me, it's all about the recording technique/mixing, etc. But in general, I would say piano.