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

Cymbals and anything by Yoko Ono.

A grand piano is a brutal test, especially for loudspeakers and phono cartridges. As the pianists hands descend down the keyboard, does the timbre and tonality of the instrument change? A very dynamic piano performance that has been well recorded is a real challenge for phono cartridges. The sound of stylus mistracking is extremely amusical.

Another is densely recorded large ensembles of either singers (as in choirs) or orchestras. How much inner detail can you hear? For great recordings of choirs (and pipe organs), look for LP’s on the Ark label, recorded by speaker designer Robert Fulton. Beautiful, delicate voices captured in very natural sound, each voice clearly separate from the others. And Robert captured the huge "shuddering" sound of pipe organs in large spaces (cathedrals), the sound of the lowest bass pedals more felt than heard (assuming the system plays that low. The 32 ft. pipe produces a 16Hz tone!)

J.Gordon Holt’s first priority in h-fi was reproducing the human voice free of what he called "vowel coloration", a term I immediately understood when I first read it. Many (most?) loudspeakers fail this test miserably. I recorded my 2 year old son's voice using a small diaphragm condenser mic plugged straight into a Revox A77 to use for loudspeaker evaluations. Talk about a brutal test!

Two channel stereo is an irretrievably broken format limited to the typical music you hear played at hifi shows (lame). A system has to be painstakingly assembled at high cost and will generally sound acceptable in only one spot in a room, the "sweet" spot.

Scrap this sham and start over with an immersive audio system.

Click the link below for more info:

Audio Engineer explains why "Why Dolby Atmos is Definitely Going to Supersede Stereo"

“We’ve been waiting for a replacement for stereo for decades,” says Jan ’Stan’ Kybert, the engineer responsible for installing the Dean St. Studios facility. “It’s an ancient technology. With music you want to feel something, like with a Saturday night or Sunday morning record. They make me feel ‘Saturday night’ or ‘Sunday morning’. That feeling has been lost with stereo now, and it’s not stereo’s fault, but with Dolby Atmos that feeling is there. It’s bigger, more exciting and wants to make you move, be more intimate, more relaxed or whatever. Everything it does it does it on a richer level.”

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.