I tend to agree that classical can be the most demanding. But not all classical is created equal.
Back when I was first auditioning Hi fidelity speakers, I always brought along a few different LPs for evaluation (this was in the time before CDs, when every self-respecting stereo store had one or two killer turntable setups). It was a real pain to get the different speakers I wanted to evaluate evenly balanced, so that the currently favored speaker (the one the store wanted to sell) wasn't louder or better equalized. But once that was done I could listen to the music
The LPs varied a bit over time, but the genres they came from never changed. The key was choosing a variety of complex music with which I was very, very familiar.
I always brought at least one popular album with well defined vocals. A regular choice was Stand Up by Jethro Tull. I brought this record because I knew it inside out, and I knew how it could sound with great speakers. I always had such an emotional response when songs like "Reason for Waiting" or "Fat Man" or "Look into the Sun" were played right. Playing this album gave me a baseline for what kind of imaging I could expect. Other favorites included Kate and Anna McGarrigle's first album, Teaser and the Firecat, Crosby, Stills, Nash & Young, or early Joni Mitchell.
Once I had an idea of imaging, and of the detail potential on relatively simple pieces, I would pull out complex classical music to see just how much information and detail a given speaker could handle. In particular, large choral pieces with full orchestras could trash many big name offerings.
Again, I chose pieces I was quite familiar with. This wasn't hard, as I sang in classical ensembles and choirs for many years. Some of my friends choose over-the-top pieces like The 1812 Overture, but that piece was more a workout for a turntable (keeping the stylus from jumping the groove) than it was for the speakers.
"Ode to Joy", from Beethoven's 9th, however, was especially brutal on speakers. Here you had Beethoven at his finest; with overlapping orchestral themes and 4 or more simultaneous voices. I remember trying it out on highly respected offerings from then-new companies Polk and Bose. Both performed so poorly on vocal details (compared to Klipsch, Kef and B&W), that I refused to take either company seriously for quite a while.