Thoughts on the most difficult instruments for speakers to reproduce?

Piano , I think the father of Bill Eggleston told him , if he can make the Andra Eggleston sound right on piano, more likely other instruments will sound right too.I own Andra , to me they sound good on piano.

I'd say a familiar voice recorded in the same location as it's played back. No futzing around with the sound via EQ and with all of its dynamics intact. 

This should also be easy enough to record and demonstrate on playback.

Slip on a blindfold and you can play 'Is it real or a recording?

I once witnessed at a show where a female vocalist was accompanied by a solo piano. The aim was not to show off any speakers but how accurately you could record live onto an SD card of all things.

I wish I'd paid more attention  (I didn't like the equipment being used) but I remember there was a difference between live and the recording but it was tiny - a hint of less depth and space.

Failing that you could try any familiar piano or violin recording which hasn't been overly bleached out with the usual proviso that it be a fairly clean and natural recording.

None of your multiple splicing editing or EQ tricks. 

If any exist.

Lots of good comments in this thread, but the one wild card in all of this is how was the recording made? What kind of microphones and preamps were used? Where were they placed? Was dynamic limiting or compression used? What other processing effects were used in the mixing and mastering process? 

Another problem is that the majority of recordings involve multiple instruments. Different instruments have radically different radiation patterns and interact differently with their acoustic environment, whether in a studio or at a live event. As such, the engineer and producer end up making compromises when choosing how to make the recording. Yes, one often sees each instrument or group of instruments miked separately, but even then they have to be mixed down into a single stereo presentation. That means compromise. Even surround sound (which is often not used with this issue in mind) involves compromises.

Given all of this, I am not surprised that I find most recordings mediocre and some downright bad. It is almost a miracle that a few recordings out there are extraordinary. There really is an art to the process that not every recording engineer and producer possesses. 

PIANO DUDE!!!!!