The one and only acoustic instrument?

That chart helps me understand why my 2-way speakers that cross over at ~ 7khz sound (blue zone on chart) so coherent and right compared to many that cross over in the yellow and green regions. That and them being omnis make for an effective combination of fundamental design advantages.

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.

The most colorful sounded instrument for me is the guitar. It might(not trying to start anything by this)be like systems. The guitar is the one instrument I can almost never tire of. For me it is not even close with any other instrument.

Oud - very acoustic with wider spectrum than the guitar.

Funny you should bring it up - The solo acoustic instument I listen to most is the Oud. However, the Piano is far more differentiating in revealing quality of the sound system, so I would listen to Piano for audition, and go back to listening to Oud for musical enjoyment.