The one and only acoustic instrument?

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.

Piano appears to be the star..thanks guys