What Instrument would you listen to?

No single instrument covers all the parameters that go into good sound, so using just one as a reference is a bad strategy unless that is all you will ever listen to. Human voice ( male and female), acoustic piano, drums, bass, violin and electronic synthesizer would cover it fairly well together.

Does anyone hear actually buy an entire rig preassembled? With a single instrument, you can get a feel for timbral accuracy, so its a good place to start, but clearly you have to move on to music that uses multiple instruments and voices to complete the evaluation.

But to answer your question. Piano first, followed in order by cello, violin, french horn, then soprano and tenor voices.
I have heard speakers that do all of these well but fail on piano. That is a deal breaker. On the other hand, I have never heard a speaker that gets piano right and fails on the rest. So the lynchpin is piano.

Piano is the instrument capable alone of getting one the furthest however.

Human voice. Spoken, sung (individually and in choir) and male and female variants.

whatever you are most familiar with. Piano is very revealing of flaws.