Piano is almost a consensus which is difficult to find in any audiophile discussion, but drums is of great interest too me as well simply because they are so hard to get right and can tell you so much about your system's performance.
What Instrument would you listen to?
Ok, this is not the real world, but if you HAD to audition a rig (for purchase) listening to only one acoustic instrument; what instrument would be your lynchpin?
Piano, for me.
Piano, for me.