probably guitar, if not piano.
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Piano, which covers a very large frequency range and has tremendous dynamics as well. A piano playing a variety of music can show you the strengths and weaknesses of a speaker system better than any other instrument, IMHO. Plus, if you know the make of piano in the recording and have heard one, it's easy to see how well a system can reproduce its timbre. My suggested recording for testing a system is the Reference Recordings "Nojima Plays Liszt", which along with being a superb performance can certainly give any system a good workout.