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

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?  Or are they just like you hear when somebody's talking to you? The same goes for saxophones.

Big Band Jazz and Romantic Era Orchestral music can be good tests, as well. Big Band can test the system's ability to punch. Classical has a good chance of showing off a system's ability to create spaciousness and a believable soundstage.

Classical.

For a long time, I wondered why folks that were into classical had the most expensive systems…. by a lot. An orchestra goes from playing a single instrument at near the level of perception to the entire orchestra overwhelming the ears to differentiate the wall of sound and everything in between. After having season tickets to a great symphony orchestra for over ten years… no question… classical.

I suspect something that Steve Hoffman said. Here I'll link it below. Something about 0:27. To this day I still have no freaking clue what he's talking about. 

https://forums.stevehoffman.tv/threads/test-the-midrange-purity-of-your-system-and-or-your-computer-speakers-using-bing-crosby.496472/

I would agree with @ghdprentice that overall, a classical symphony is the biggest challenge. But as I do not listen nor attend a lot of classical music, I use piano and vocals as my “challenge” for a system, as I am very familiar with many vocals and what a piano should sound like.