For me, the Kyrie from Tomás Luis de Victoria's Officium Defunctorum (1605) is the best piece of music ever written, period. And I say this after having listened to a lot of music of various styles.

Here's the piece with two great performers:

- The Sixteen: https://www.youtube.com/watch?v=MO-C_zC6KiQ

- The Tallis Scholars: https://www.youtube.com/watch?v=KOko0MqkNcc

Enjoy!