Johannes Ockeghem: Missa Pro Defunctis

Missa Pro Defunctis (or Mass for the Dead) is one of the earliest polyphonic pieces of music to have survived to this day.  It is thought to have been written between 1410 and 1497, which is just really cool to think about.  It's simply awe inspiring.  I love turning this up as loud as I can take it and letting it shake my very soul.