Aside form the Human voice...


what is the most expressive instrument? I say it's the Electric guitar(natrual and artificial harmonics etc) followed by the Pedal Steel.
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Guitar can't play 10 notes at once, can't....on and on...doesn't mean it's not more expressive, but it is more limited. I vote Piano.
Hell, if it's the number of notes and different voices at once that you want, how about the organ? In the right hands (and feet) it can be very expressive. I still vote for the cello, though.
I think it's down to the musician, not the instrument.

Most wind instruments can't be polyphonic, and most stringed instruments (and I'm including piano) can't be anywhere near as expressive on simple music as a wind instrument.
Every voice and instrument has it's own unique expression. I have a fondness for a well recorded vibraphone,i.e Lionel Hampton style.