Depends on the occasion. Ockeghem of course, but for practical purposes where a mixed choir is available only Victoria will do (by some counts that's already 3). I have to admit I'm not familiar yet with Lasso or Palestrina, but among low voice Requiems Morales and Lobo are slightly outdistanced by LaRue. CPDL has 41 works, found with a multi-category search.
The best is the Richafort a6 setting, based on a Josquin chanson and written in memoriam of Josquin at his death. It is spectacularly, sublimely beautiful.
There are a few recordings of the complete work on YouTube, as well my own recording of the Lux aeterna.
Palestrina‘s „Missa Pro Defunctis“, performed by Chanticleer. The Requiem by Morales comes in an easy second for me. I haven’t explored too many others but I saw Victoria‘s on CPDL and the music looked relatively simplistic to me, but I haven’t listened to a recording yet.
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