If you could just pick one instrument that you think sounds best on your speakers

Other than voice, I like guitars-dobros, acoustic, banjos, mandolins and fiddles. B&W's from 600 series from around turn of century and 685 S2's.

For my CLS speakers it has to be the violin (or any stringed instrument). I swear you just about hear the rosin flaking off the bow!

For my main system (Wilson Maxx 2) while everything is well rendered, the low bass, particularly drum, sounds better in a way that few experience. I never realized that most speakers render low bass imperfectly, with definite overhang, until I listened to the bass on the Wilsons. You hear the drum, and then it stops, No ringing (I'm not talking about reflected sound that come a fraction of a second later, but a situation where the main note slurs a little and hangs on for a fraction of a second).

You can experience the same thing at a classical music performance in a hall without proper sound treatment vs. one that has.

Guitar!

Because it could be electric, acoustic, 12-string, slide, steel, lap steel, baritone, bass,....

Need I go on?

I would have said vocals; but the number of tracks of unaccompanied vocals I love is infinitesimal compared to solo guitar.

Acoustic guitar.

Piano. Focal Electra 1007 Be standmounts