What Instrument would you listen to?

Piano it is. Thanks for playing ball with me

I remember the sales guys in the shops being proud of certain speakers capable of reproducing piano exceptionally well. It's not an attribute one would have to discover on their own. So I think a well informed individual would by necessity choose piano firstly.

If it has to be just one instrument, I agree it would be piano.
Two allowed it would be piano and violin.

Saxophone would almost seem like a good choice, the problem is it can have such a huge variety of sound one can hardly be sure what is the 'definitive' sound. So that rules it out as a great choice for auditioning.
(I have never used a saxophone recording to audition, even though it is one of my favorite instruments!!)
I use stuff with choral, female vocal, orchestral, piano, big/lots of drumming, piano/violin

that's easy- the piccolo.

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.