J.S. Bach - for the aching lyricism and the polyphony and the unexpectedness and the sheer weight of his personality. Particularly the cantatas - Ich will den Kreuztab, Ich habe genug, Ein Feste Burg ist unser Gott, Erschallet ihr Lieder and so on for 200 works. I also love to play the 2 part inventions on the piano, never get tired of them. Aiming at being able to play the 3 part sinfonias soon. Oh and there are the Passions and the B Minor Mass. And more, much more.
Franz Schubert. Another extremely prolific composer so it's good to fall in love with him too. He does aching lyricism as well. Outdoes anybody for lyricism actually and for melody but can do a wide variety of emotions - try
http://www.youtube.com/watch?v=P5B6nysheec for a great interpretation of his most famous song. And there are 600 more of them. Wonderful late piano sonatas - dramatic and touching - and the chamber music. He was getting better and better all the time but was dead at 31.
Robert Schumann - mostly for his songs but also piano pieces. All the manic-depressive emotion you could possibly want.
Debussy. The piano pieces shimmer in no particular key. Ravishing. Try Images and the Preludes.
Hugo Wolf (no greater songs exist according to Harold Schonberg, the late music critic for the NY Times)
Brahms. If you like rich, deep sound and lots of development.
Haydn...Richard Strauss...Purcell...the English and Italian madrigalists....Arvo Part for luscious sound that doesn't require too much input from the listener...Mozart for the operas and the piano concertos...
Got to stop.