Thoughts on the most difficult instruments for speakers to reproduce?

Piano.

Digital is not always bad, so with the proper set-up we can get quite acceptable quality, meaning a lot of instruments sound very acceptable. Of course, the rest of the system should support such elevated quality levels as well. Nevertheless, even with the current high end audio, it is still easy to differentiate life from recorded music. (analog and/or digital). It al comes down to what we find acceptable, which is personal. Some people are prepared to go further than others. I believe that the sound of certain instruments is harder to reproduce using audio equipment within a certain price class. To reproduce the sound of certain instruments properly, it may take a higher investment, perfect room acoustics and so on.... (however, I (and you) believe I (we) can easily detect if what I hear if life or recorded music, so...the engineers are not yet done with it...)

Drum kit

xylophone
upper octaves of the piano

Yes speakers and equipment have limitations. But Its the recording that is often the real limitation. Several on this thread stated that the speakers can compress the sound of instruments like violin, piano, or just a simple cymbal crash. Speakers are not always the root cause. As a drummer I know most recordings of percussion are not captured at full dynamics. Other instruments have the same issue. If it's not on the recording how are you going to play it back. So much of what you hear live or in a studio still isn't captured by mainstream recordings. What's the hardest instruments to reproduce? Any instrument that wasn't fully captured.