If you want to improve your playing, there are a few musts:
Firstly, Bach is in my opinion the best composer to improve technique. Careful study of Bach's keyboard music will result in excellent co-ordination and evenness, plus his music is among the best written for the piano. Start off with the Well Tempered Clavier; this is a good introduction to contrapuntal music, and the preludes focus on aspects such as apreggios and chords. After you've mastered this, the Italian Concerto is a must. It's also a wonderful piece of music. If you're ready after this, the Goldberg Variations are a very challenging but rewarding collection. Bach is probably the best Baroque composer to study.
After this, it would be a good idea to play some Classical music. Haydn is an often overlooked yet very helpful composer, and his 50+ sonatas will help with evenness and accuracy.
Mozart is a must; definitely one of the most helpful composers. Some of his music can be rather dull at times, but it's still worth learning.
Once you've become more advanced, Beethoven's piano sonatas will be an excellent addition to your repertoire. They are technically difficult at times.
Now some Romantic composers would be helpful. Chopin, the greatest composer of this period, provides music that is stimulating and wonderful to play. It is really enjoyable to go through a few nocturnes and really express yourself. The 24 preludes are the perfect starting point; most are fairly accessible, with a few more difficult and some (ie. the prelude in F# Minor) are at a level of virtuosity that few ever reach (avoid these ones!!). After this, the nocturnes and mazurkas are musts. Avoid the Etudes and Ballades until you are very advanced.
The piano sonatas of Schubert are wonderful; as are the illustrative pieces of Schumann and Grieg. Brahms' waltzes are worthwhile.
Bartok is a good modern composer to study. As someone said earlier, Uematsu's music is wonderful.
Also, I advise that you STEER CLEAR of Hanon's exercises. They are useless.
Finally, don't touch anything by Lizst or Rachmaninov until you are extremely advanced. You could seriously hurt yourself!
Hope this helps...