Since the thread keeps being "the most favourite choral work" and since "Requiems" seem to be an easy way out in preference, some of the least known on the two sides of the spectrum:
From the very early ones, you may try the newly released by Hyperion Richafort's Requiem. A work of luxuriant and spectacular choral writing leading to the metaphysic and meditation.
In the same period, Du Caurroy's Requiem: Excess of austerity, but with a large palette of vocal and instrumental beauty.
For the more contemporary (for our Clive):
Try Greif's Requiem (of 1999): a modern masterpiece!
In the same vein, Bussotti's The Rara Requiem (1969): a meditation on the beauty and the fragility, the life and the death through the charm of music, the mystery and the quality of the timbres and the sonorities. All these in the austerity of a writing strictly atonal!
There are some thousands of Requiems only. As for the Masses, forget it. Life is short.