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

You should play the music YOU LIKE!!  

I don't care one whit how opera sounds, because I hate opera!!  Some of the best sounding new music made today is country, but I hate country music too!  I love 80's synth pop and rock, so my system better make that sound real good.

And then there's always Steely Dan!!  

Agree on classical. However, if you actually attend a concert, it’s not really that loud. Depends on the piece, but blasting some crescendos isn’t going to tell you much. I’d go for organ. It’s actually the first hi-fi rig. It’s listening to how it reproduces ranges of textures, tones,  transitions from midrange to bass. And the same recording on two different organs should be radically different because of reproducing the space. 

Also, play something groovy. If you move, tap toes, etc. that’s the point. If you hear a band live that just isn’t getting you to move something is holding it back. Amplification probably is a bigger source for this but lack of decent bass too. Or some call it PRAT. Things out of time are easy to tell. They just don’t swing. 

Something I know and like that is well recorded.

It clearly depends on what music you listen to the most, and what you want from your system.  I am a lover of jazz and chamber music.  I've recorded and attended concerts and know what full orchestra dynamics are, and I' recorded and listened to pipe organs.  So here is my take on these.....which ranking or preference you have is up to you.  

1.  Vocals .... Sinatra and Bonnie Riatt are my favorates.  The voices should sound like voices.... no sibilants, mistracking or any obvious reproduction issues.  The vocals should have "presence" but the tonal balance has to be highly agreeable to you.  Otherwise very good systems can add a little tenor emphasis or a little alto emphasis to both their (and many other voices).  But in either case the voices must sound natural.

2. Jazz .... well recorded club music.  Make sure their is both width and depth to the sound.  As somebody stated earlier, make sure the piano sounds "all of a piece" top-to-bottom. Likewise the string bass....plucked strings should sound "all of a piece" dispite the disparate range. Brass should have no tracking issues.  There is no substitute for attending a fair number of jazz sessions in different club environments to train your ears to these sounds.   You might need to use several disks as references, since club recordings are difficult to record really well.

3.  Chamber music ... evaluate much like jazz, live recordings if possible.  Sit a few times in the closest row; a few times further back with some hall sound.  Get familiar with both.  Chamber music can be recorded many ways....some natural and some simply mono-mic'd.

4.  Symphonic Orchestra ... make sure that you are using recordints that actuall allow the hall acoustics to be heard.  Know the type of gear used (older classical music from the 50's, 60's, early '70's often showed photos/described the recording set up.  Know what an ORTF mike setup sounds like; know what a three channel mono setup sounds like.  Make sure your amp/speaker combination can handle the loudest sections without cartridge breakup or speaker breakup at the loudest levels you will listen.

5.  Pipe Organ - Get a recording or two from a church or cathederal playing music that has 32hz pedal notes....these should be solid and if you are on a wooden floor, should usually create a vibration as well (below 32hz you will mostly "feel" vibration).

*********************************

Hope this provides help in one way or the other for some folks here.

Wagner absolutely. For example the last ten minutes of Siegfried.