The true musical answer is - all of the ones included in a single musical piece -
Musicians have argued this for many a long year. But all agree if a piece is written for all the instrument in the orchestra and one fails to play then it is noticeable. therefore every instrument that the composer wrote the piece for is vital.
The right answer is Mandolin
Piccolo, though some percussion instruments can be smaller.
The piano and the violin were the most popular solo instruments used in the concerto of classical period. The violin was one of the most popular for the baroque period, and the increase in popularity of the piano caused that this instrument replaced the former. See http://au.encarta.msn.com/encnet/refpages/RefArticle.aspx?refid=761553032
In a string orchestra, the instrument with the lowest pitch is the Bass (also called Double Bass, String Bass, Upright Bass, and Contrabass)
The London Symphony Orchestra
violin
writing
The leader of the orchestra - its a violin from the front desk.
The quietest instrument in an orchestra is the triangle.
The saxophone is not typically a standard instrument in a traditional orchestra, but it can be featured in certain compositions. Its importance in an orchestra depends on the specific piece being performed.
The violin is the smallest stringed instrument in an orchestra.
The instrument in an orchestra that has the highest pitch is the violin.
Because Within the orchestra the piano usually supports the harmony, but it has another role as a solo instrument (an instrument that plays by itself), playing both melody and harmony.
By numbers, it's the string section. A full-sized orchestra has around 60 string players
By numbers, second place goes to the violas.
conductor
Mozart composed in every form of the time, but he probably composed the most for the orchestra.