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

synthesizer music, Larry Fast, Jean Michael Jarre, Mike Oldfield.

The most challenging anything to do it that which you do not like. Polka? @elliottbnewcombjr  glad you are still here, you seemed pretty down the other day 

A wide variety of non-audiophile records. You want to hear the shortcomings, where the illusion fails. Something that is demanding on bass will not reveal midrange anomalies and vice-versa.  

I recall decades ago Bud Fried(IMF and Fried speakers) told me the hardest instruments to get right in order are male voice, female voice and piano. Voice because we are so familiar with it and male voice is harder because it goes lower than female where speakers often do poorly. Piano because it is wildly dynamic, sudden peaks of 39 dB and more. So I would find opera with both voices and and orchestra to reproduce and separateerful choice. and a powerful piano concerto another pow

To my ears the more resolving the stereo is, the harder rock is to play at louder levels. Distorted guitars come off as grating in many recordings from the decades that rock and roll dominated the airwaves. In the 70s and 80s most of this music was recorded nearfield and optimized for auto radio play. This also caused poor bass recording. Some of the newer rock recordings have steered towards a more audiophile recording mentality. Examples... Nothing but Thieves / Broken Machine and Chevelle / Niratias. I use equalization in my system to add bass and smooth grating higher frequencies in many older rock recordings and bypass the e.q. when not needed.