Thoughts on the most difficult instruments for speakers to reproduce?

Digital recordings are a terrible weakness of my system. Far too stucotto.

The obvious/tried-and-tested options are all mentioned above (ie, piano; massed strings; muted trumpet). 

But another instrument that tests speakers/entire system in a very different way is the vibraphone ("vibes"). This is primarily played in acoustic jazz and/or modern percussion composition.

The layout of brass keys is piano-like, but the sound is very different: keys are struck with mallets, so you get that 1st-strike transient, followed by lengthy sustain (courtesy of sound propagation tubes beneath each key). My favorite players use 2 mallets per hand & can play many notes, simultaneously or in fast sequence.

The result is a characteristic "shimmer" to the sound. It's a louder than you think, something microphones readily react to if not set up properly. 

I find that speakers & electronics that fully convey the complex sound of this instrument can generally hit it out of the park on just about every instrument.

Massed (not single) violins, I agree, and (surprised no one mentioned) full bodied, contrapuntal choral singing.

My wife plays piano. I am a lapsed student of the instrument.

I put on some audiophile record (forget what it was, but it had excellent piano reproduction) on the big system and she called from the other room - "Is that YOU??"  It sounded so much like her piano in the next room.

Most of the time recorded piano doesn't really sound like a real piano played (even in the next room).

I tried it later on my Electrostat system and it works there too - pretty sure it wouldn't if it was a hybrid speaker mating a cone woofer with the main panels.

Mrdecibel...Thanks for the tip on Miles's " A Tribute To Jack Johnson, very good.

Glad to see that this fairly long chain has remained so informative and positive. Amazing that it hasn't morphed into a hardware p*match (no hardware mentions at all!), I knew it was possible on the forum!

Hmmm...should I say something about cable snake oil and see what happens? J/K