Kyrie from Missa Mater Patris - Josquin des Prez (performed by Chanticleer)

The Missa Mater Patris by Josquin des Prez is my favorite mass, and the Kyrie (below) is my favorite part.  To hear the entire mass, which is ~1/2 hour, check out this playlist.  This recording is by the group Chanticleer - sort of the American equivalent to the King's Singers.

(This is actually a "parody mass" - for more info, click here.)