It's the violin, piano is rarely used in orchestra pieces.

He wrote piano music, solo (with piano accompaniment) pieces for violin and cello. He wrote pieces with orchestral accompaniment for piano, violin, and cello. He wrote chamber music for string quartet and for piano trio (piano, violin and cello). Most of his music is orchestral and operas.

Violin and piano

Violin cello and a piano

Actually, a concerto can be for almost any solo instrument and orchestra. The commonest are those for piano & orchestra, violin & orchestra, and cello & orchestra.

Guitar!(: