It's a huge category. Do you have a favorite era?
Maybe some Monteverdi?
Byrd or Taverner (english polyphony is amazing - particularly for audiophiles)?
Bach or Handel (it's almost Christmas!)?
Mozart Masses or Requiem
Beethoven's Missa Solemnis.
The Berlioz Requiem, I have a soft spot for both Cherubini's and Saint-Saen's Requiem.
Orff, Carmina Burana (even the bits beyond the horror music)
Not a choral choice, but Brahms' Alto Rhapsody (for soloist and male chorus) is one of the greatest pieces of music ever written. Get the old Klemperer version. Brahms has a Requiem too, but I seem to be a little requiem heavy here.
Gregg
Forgot to add, if you're into classical music and have never heard a choral work by Bruckner, and are familiar with his symphonies, it's an amazing experience to hear how he gets the choral sections to fit his orchestral world.
