As a church organist, I have to throw my two cents in.

Durufle Requiem-IMHO, the most beautiful setting of the requiem out there. Be sure to get the Robert Shaw recording with the Atlanta Symphony.

R. Vaughan Williams, A Sea Symphony-Very exciting, you need the Andrew Davis recording, with the waves and shore on the cover.

You can't go wrong with Bach or Mendelssohn organ works, just make sure you have your subwoofer on for those 32' pedal notes.