Cross-posted from a recent post of mine on the Image Journal Forum, in response to someone who was asking for recommendations of choral music, particularly masses or Orthodox music.
Essential complete works (my favorites marked with an asterisk):
*Bach B Minor Mass
*Bach St. Matthew Passion
*Bach Christmas Oratorio
Bach Cantatas (there are nearly 200 of them, so check out this site for a listener's guide and specific recommendations)
Beethoven Missa Solemnis
*Beethoven Mass in C
*Brahams Ein Deutsches Requiem (German Requiem)
Dvorak Stabat Mater
*Gounod Messe Solennelle De Sainte Cecile (St. Cecilia Mass)
Haydn The Creation
Haydn Nelson Mass
Mozart Coronation Mass
Mozart Mass in C minor
*Orff Carmina Burana (the only non-sacred music in the bunch, but a great piece, which would be on most lists of essential choral music)
Schubert Masses No. 2, 3, 4, 6
*Tallis Spem in alium
*Tallis Complete English Anthems
Here is a stellar recording of selected shorter sacred choral works, absolutely sublime, probably among the top 5 CDs in all my collection:
A remarkable CD, Officium, performed by Jan Garbarek with the Hilliard Ensemble, blends haunting polyphonal sacred choral music with a solo saxophone voice. Beautiful.
And for your Orthodox ear, not choral music per se, but these are the big three contemporary Orthodox composers, known as the "Holy Minimalists":
If you want to stick with just masses/choral music from among them, try
Arvo Pärt's Berliner Messe, Magnificat, and De Profundis, all available (among other works) on this CD.
Sorry I don't have a great familiarity with older Orthodox choral music. You might find this blog post, by someone who seems to know what he (or she) is talking about, of use.
See also Top 10 Choral Works to Start Your Classical Music CD Collection (includes a couple of works I wasn't familiar with).