Bach was most famous as an organist.
Bach's most famous song is Toccata and Fugue in D Minor.
Perhaps St. Matthew Passion or Mass in B Minor.
bach was famous for his compositions.
Bach never composed any opera's. He did compose cantata's, however.
Mozart and Bach are two.
Most all of his life (1685-1750)
Johannes Bach
The most famous theme and variations ever written is Bach's The Art of The Variation (also known as The Goldberg Variations). MP3 and MIDIs of this work can be obtained at: http://www.sdrodrian.com/bach/the_goldberg_variations/
Although many of Bach's works are "famous" (well known throughout generations including Bach's own and into our own), I think it may be argued that the Toccata and Fugue in d minor for organ is probably one of the more famous works now due to its inclusion, iconification, and proliferation in modern media like film, television, and radio.
there were no pianos when JS was alive
Sebastian Bach