The "Maurerische Trauermusik" (Masonic Funeral Music) certainly is one of the highlights in Mozart's music. He not only combines contrapuntal techniques, but it is one of his rarely composed cantus-firmus pieces. You'll find a Gregorian choral in long notes in it, which origin is unclear. It is assumed that it is a choral related to the fremasons - as the music was composed for the death of two of their members.
Edition Berliner Bach Akademie