Bach's B minor mass (of which this Agnus Dei is a part) was hailed in the 19th century as "the greatest artwork of all times and all people". To this day it is widely acknowledged as one of the greatest monuments of western culture. The work was completed in 1749, a year before the composer's death, and is one of Bach's last works, though much of the music was written earlier or adapted from his other works. It was not performed in its entirety until over 100 years after Bach's death. In Beethoven's lifetime the work was already so legendary, despite not having yet received any performance, that Beethoven tried twice (unsuccessfully) to acquire a score. This performance, by Janet Baker, is for me one of the most moving.
J.S. Bach's Agnus Dei from his B minor mass, sung by Janet Baker with the New Philharmonia Orchestra conducted by Otto Klemperer (recorded October 1967).