Mozart's Requiem, one of the greatest compositions in the Western musical canon. The version I'm listening to is by Sir Colin Davis and the London Symphony Orchestra and Chorus, but there are many great recordings around. I know Ben Stein is very partial to a rendition by Sir Neville Marriner and the Academy of St. Martin-in-the-Fields (maybe it's this one). (Marriner was, coincidentally, music director for several years of our very own Minnesota Orchestra.) There's always a section in the Dias Irae, the Lacrimosa (Ah! that day of tears and mourning!/From the dust of earth returning/man for judgment must prepare him;/Spare, O God, in mercy spare him!), that makes me stop whatever I'm doing and just listen.
One of the most profound days of the liturgical year is All Souls' Day, November 2, which is always commemorated at St. Agnes (and other Catholic churches, in fairness) with a Requiem High Mass, featuring the Mozart Requiem. To listen to the pleadings of the Dias Irae, the solumnity of the black vestments, and the sublime genius of Mozart, is an experience to last the entire year.