Johannes Ockeghem
Ockeghem was not a composer who wrote lots of music, but it is hard to tell as some of it might have got lost. There are 14 masses that survive and other vocal works. His Missa pro Defunctis is the earliest surviving polyphonic Requiem. Ockeghem based many of his masses on well-known tunes in a way that is called “cantus firmus technique”. Sometimes he put the borrowed tunes in the lowest part (he sang bass himself). This was rather unusual.