French Horn (do you hear the fanfare?)! Combining a 4-1/2 octave range with incredible harmonic richness, a horn has a wonderful diversity of possible sounds. Brassy, lush, refined, romantic, stirring... What other instrument can migrate from the brass section in a band or orchestra to a woodwind quintet. You know you have a decent system if it can get the horn sound right. I tend to go for the classic Dennis Brain recordings, but there are many good players today, each with a unique sound.
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.