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

Why do you say Cello?
Number of notes wasnt criteria. Previous post said guitar could do everything piano could. I listed one thing it couldn't do.
lets offer up jimi hendrix as evidence. there are lots of great 'expressive pianists' but jeez louise, to this day what he did with an electric guitar is still looked upon as more than about magic?
Tabs, if you listen to the Rachmaninoff cello sonata, or Bruch's Kol Nidrei, or the Elgar cello concerto, you'll see the vast range of emotion that can be brought out of a cello. Maybe leaning towards the melancholy a little, but it just strikes home to me. It resonates so beautifully too--I won a cello lesson from one of our NJSO musicians in a silent auction and got to play it for the first time, you can't believe how it resonates through you when you play it. And it's not just the notes (the lows and the highs) and the ability to bend and finesse them like you can with the guitar, the bowing gives the artist the ability to play with phrasing, both in timing and in terms of volume and texture. You could say the same about a violin in most ways, but it doesn't reach the low notes that stir the soul (or at least mine)....

Nuguy, I have a record of the Temple City Kazoo Orchestra, to say that listening to them play is an emotional experience is an understatement.