In reply to flash13:
Recommending some classic music is like recommending food, there is a huge variety, and everyone will favour something else. So, just off the top of my head, and trying to include some representative works for a few different eras:
Claudio Monteverdi - Vespra della beata vergine, e.g.
Heinrich Schütz - Geistliche Vokalmusik, Saechsisches Vocalensemble / Matthias Jung
J.S. Bach - Art of Fugue, piano version by Pierre Laurent Aimard. It is a fantastic, meditative and intellectual exercise, but requires repeated hearing.
J.S. Bach - St Matthew´s passion - a great recording for free on the all of Bach website, otherwise the 800 year anniversary recording of the Thomaner choir.
Händel - Giulio Cesare in Egitto, the Mortensen recording from Copenhagen, look for the DVD on youtube
C.P.E Bach - Magnificat (way better than his father´s!)
Mozart - Zauberflöte
Beethoven - Rasumowsky-Quartet, Symphony #3 (Eroica)
Schubert -Unfininished Symphony
Brahms - Ein Deutsches Requiem, e.g. Schönberg Ensemble / Harnoncourt
Dvorak - Cello concerto, I like the Janos Starker recording
Strauss - Eine Alpensypmphony (must hear for climbers, it actually describes climbing a mountain!)
Max Bruch - Kol Nidrei, Starker again, also his Eight pieces for Clarinet, Piano, and Viola
Sibelius - Violin Concerto, Valse triste
H.M Gorecki - Symphony #3 (Symphony of Sorrowful Songs)
I am not a great Mozart fan, someone else should offer their favourites...
CB