Bach's Toccata and Fugue in d minor is one of the most beautiful pieces I've heard. Every time I listen to it, I hear something new in it.
Capriccio Espagnol by Rimsky-Korsakov is another excellent one, as well as the Butterfly Lovers' Violin Concerto. Elgar is my favorite classical composer, from the Pomp and Circumstance marches to Salut d'Amour. In piano music, Beethoven's Pathetique Sonata is my favorite, with Rhapsody in Blue as a close second.
There was a symphony and each part of it was a different animal. The sounds each described different animals. For example, an elephant would be deep and slow, and birds would be high and fast. Does anybody know who wrote it, or what its called?