Final answer:
Johann Sebastian Bach is the composer who wrote significant works for organ, keyboard, orchestra, and small ensembles, and is known for his influence in the Baroque period of music.
Step-by-step explanation:
The composer responsible for writing numerous organ and other keyboard works, as well as many pieces for orchestra and small ensembles, is Johann Sebastian Bach.
Bach was a prolific German composer and musician of the Baroque period, and his contributions to the repertoire of keyboard music, including his well-known works such as the Brandenburg Concertos, The Well-Tempered Clavier, and various cantatas, have been celebrated for their artistic and technical mastery. Among the listed options, Johann Sebastian Bach is best known for his extensive output in these genres.