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!