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

1.  Music I'm familiar with.

2.  Solo piano.

Tonality - Human Voice. Male or female, It shouldn't sound tubby, chesty, cupped, or nasal, Emmy Lou Harris Eva Cassidy, Aaron Neville, Mark Knopfler (His voice turns to mud on a lot of systems)

Dynamics - A big symphony, Beethoven's 9th is the first that comes to mind

Detail - Acoustic guitar. Pat Metheny and Charlie Haden. "Beyond The Missouri Sky"

Soundstage - Any well recorded live album, regardless of genre, classical, jazz, rock. Dan Hicks "Where's The Money" on a goodsystem will put you in The Troubador circa 1972. The first time I ever heard Magneplanar Tympani IIIs biamped with Audio Research D-76s and D52s with this album was life changing.

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.