Thoughts on the most difficult instruments for speakers to reproduce?

Its not just speakers. These things are difficult for every component in the chain. As my system has gotten much better over the past year or so one of the best most unexpected surprises has been the way every recording sounds so much better now. Every single one. So if you have one that's a "tweeter killer" I would look long and hard at what else might be going on there.

 "Don’t laugh but I would always bring the ultimate tweeter killer speaker shopping with me, BST Spinning Wheel."

SW is an excellent demo song. I have a first run press(XSM137878) along with a MONO South American press(not rare, but unusual)

The mono press actually sounds  more convincingly "real" to my ears. No instruments/vocal leaning left or right. Great album.

I heard a test LP of SW on a UBER $how system. Those horns can sound pretty real with the right stuff.

My now possibly obsolete Klipsch Heresy IIIs are amazingly realistic and coherent, and I listen to piano stuff a LOT. Good gear upstream is key for this of course, but piano sounds great unless it's a crappy recording.