Debussy and Bach are my favorites.
Debussy's Noctures and La Mer are great listens.
Right now on piano, I am learning Claire de Lune. Its hard, I'm by no means an expert at piano.
My favorite Bach piece is Toccatta and Fugue in D minor. I can play 4 pages of it but after that it gets insane hard.
His organ fugues "Little" and "Great" are really good.
Holst's "The Planets" is awsome.
I love Beethoven's Moonlight Sonata, mostly the really fast 3rd movement.
Griegs Peer Gynt Suites I & II are both good
Tchaikovsky's Swan Lake is excellent, as is The Nutcracker Suite, my favorite of that being Arabian Dance.
Chopin's Prelude No. 15 in D flat major is excellent.
Vivaldi's (sp?) The Four Seasons is good too
And this whole thread is just scratching the surface! Classical music is my favorite.