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.
128x128tabl10s
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.
Strings that can be bent provide additional range for expression(try that on a tradional piano).

Rcp
Why do you say Cello?