The final work Bach completed was a chorale prelude for organ, dictated to his son-in-law, Johann Altnikol, from his deathbed. Entitled Vor deinen Thron tret ich hiermit (Before thy throne I now appear, BWV 668a). Source: http://en.wikipedia.org/wiki/Bach

A minor

Beethoven's last completed piece was the replacement finale to his string quartet Op. 130 in B-flat. It was written as a substitute for the Grosse Fuge, which some of his friends found too long and weighty.

A composition for an organ that incorporates a hymn tune is known as a chorale prelude.

Take your pick! Extant forms when Bach began composing included the Prelude, the Fugue, the Chorale Prelude, the Toccata, several concertante forms, and so on. Bach raised all of them to their highest peak. There is no one best answer, but the "safe" answer is probably "Chorale Prelude".