Top billing goes to the Requiem, Ockeghem's most widely recorded work, and perhaps his most enigmatic piece, stylistically very wide-ranging and diverse. Aesthetic judgement is hard to pass, since it may well be incomplete; but the surviving movements contain some of his most arresting inspirations. This is the first version of any quality to feature sopranos on the top lines. Incidentally, no recording of the Requiem is uniformly excellent; on the other hand, the words of the Mass for the dead conjure up many associations, and The Clerks deserve praise for the verve and imagination with which they respond to the work's interpretative challenges.
The fillers are the works built on Ockeghem's song Fors seulement (which includes Antoine Brumel's Du tout plongiet). It's difficult to decide which to praise more highly: the pieces themselves, which are incomparable, or the singing, which represents The Clerks' finest achievement to date. Fors seulement inspired a flowering of astonishing pieces scored for very low voices (initiated, it appears, by the composer himself): in both the Mass and in Du toutplongiet, the basses descendto written low Cs.
In addition, these pieces are exceptionally richly scored (the Mass and the La Rue song are five-voice works), creating polyphony as dense and as dark as a strong Trappist ale. The Clerks achieve almost miraculous linear definition here, without losing an iota of the music's sensuous appeal: that's quite a feat, given the low pitch and awesome contrapuntal complexity involved. This must be counted as a major achievement.

Enough superlatives: this is a major achievement.