There are a number: the human voice is probably key because there are easy live references for that, also piano, vibes, violin, crash cymbals, massed brass.
The second question IMO is quite irrelevant; the answer is the system and its setup. The "best" speaker in the world connected to a system with weaknesses will be dramatically inferior to a properly tweaked junk speaker hooked up to a replay system which has been completely sorted out, as far as the ear is concerned. Yes, the latter will probably have frequency response deficiencies, etc, but from the point of view of the listening experience, there will be no comparison ...
Frank
I second the human voice mention here by Frank, it is when one is "catched" by a realistic reproduction of vocals when a systems gets full attention... At least this worked for me.