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.
The cello, in my view. Guess the fact I'm a bass/baritone has a lot to do with it.
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If only the piano had a wind controller on it ... In lieu of that I would say the voice, then most wind instruments like sax and oboe and such, then a tie between piano and other stringed instruments.

It still takes a human touch to make real music (thank God).

piano, IMHO--- the only instrument with two voices that can play at the same time. can't emote more than that. Ya hear that Elizabeth?
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.
"01-19-07: Tabl10s
The guitar can do everything the piano can...and more.
Tabl10s (System | Threads | Answers)"

Not really, the freq range that a piano can cover is much wider than guitar.

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.
Number of notes wasnt criteria. Previous post said guitar could do everything piano could. I listed one thing it couldn't do.
Strings that can be bent provide additional range for expression(try that on a tradional piano).

Why do you say Cello?
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.
Given Ojgalli's caveat, and assuming an equally expressive musician per instrument, which instrument has the most room for expression? I would agree with Rcprince and say cello (and a close second probably goes to ptmconsulting's sax suggestion).

The big "problem" with piano (and therefore organ) IMO is that it is percussive only. Sax has the wind control (and reed control on top of that) which allows for continuity of varying types/degrees (with discrete note separation or note melding using "glissando"). It's in the same general register as the human voice, and covers a wide (3) octave range with a tone ranging from sweet to raunchy. However, sax is not percussive. Cello probably has a slightly wider range (usually 4 octaves) and notes are either discrete or completely run-together. Cello lacks the ability to get super "raunchy" the way saxophone can but the player can both 'push' and 'pull' the instrument for richness of tone, dynamic control is far better, vibrato is probably best in the orchestra, and a single player can be either Isaac Freeman (the bass for the Fairfield Four) or the sweetest, most delicate alto, AND everything in between.

rcprince, I completely agree about the Elgar and Bruch. Among expressive cello, I also like Peter Wispelwey's playing of Benjamin Britten Cello Suites on Channel Classics, Mario Brunello's odd compilation called "Alone" (tough to find in the US sometimes), Schelomo Rhapsody by Ernst Bloch (same dark vein as the Bruch), and Edgar Meyer's rendition of Bach Unaccompanied Cello Suites (on double bass), which is a something of a shocker if you find low notes stir the soul. The last stays high on my playlist no matter how many times I consciously try to "take a vacation" from it.
T-bone brings up an important question. What expressive techniques do musicians have available on different instruments? Earlier in the thread frequency range was sighted. In addition there is dynamic range, tonal & timbre range, articulation, pitch control/vibrato, and phrasing. All these things contribute to the possibilities of expressive control by the artist. Different instruments have different amounts and ways of applying these techniques. For example, prepared piano, or plucking, damping, striking the strings with implements, expands the expressive range of the piano. Mutes expand the range of brass instruments. Bowing techniques add expressive range to strings and tonguing techniques to winds. Add breath control, electronic and computer manipulations, different stops for the organ, double stops, triple stops, polyphonics, subharmonics, overtones, plus an array of other experimental techniques and there opens up a plethora of potential. Even if we disregard all the 'artificial' means, there's still the subtle, elusive, yet endless, possibilities of phrasing. Even an electric organ, possibly one of the most mechanical instruments, in the hands of one musician can set the house on fire, and in another's, leave us yawning.
T-bone, I have the Channel Classics SACD, that is indeed a nice one, and I'll have to look for the Edgar Meyer recording. And if you haven't heard the Rachmaninoff, I think you might like it--the Steven Kates version on the Sonic Arts label from way back is a nice performance and recording (it's the one that introduced me to the piece), and there are some others out there as well.
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.
rcprince, Edgar Meyer can make a double bass sound more delicate than I thought possible, especially in the upper registers.

2muse, I agree the French Horn is a nice instrument (and for percussives, you can always tap the side of a french horn with a drumstick)!