Mozart's Requiem Mass is considered one of the greatest choral masterworks of all time.  It will bring tears to your eyes, and joy to your heart.  From the chilling Dies Irae to the ethereal Benedictus, this work should be a part of your community group and your personal collection.  Here available on ilearnmusic.com is the complete vocal score with keyboard reduction of the orchestra.  Also, check out the other links below:

An article regarding the history of Mozart's Requiem...

Download the complete vocal score with keyboard reduction
(PDF Format - Warning: it's a large file - over 5 MB!)