I would suggest the following:At least one early Beethoven Sonata, one from his middle period and, if you are up to it, a later Sonata as well.I agree with the WTC suggestion.Chopin Etudes, but first get a handle on a Scherzo or a Ballade (4 is a beast, 1 is tough, 2 and 3 are easier but still full of good music). A handful of Nocturnes too.MOZART! You have to learn at least 2-3 Sonatas.Debussy Preludes will help round out a pianist too. And, come to think of it, pick up a Brahms Rhapsody (Bm is good - not that difficult but a good introduction to Brahms.
Bach WTC, Beethoven sonatas, and Chopin mazurkas.*record scratch*Why Chopin mazurkas, you say?Because it allows you to focus purely on musicality; this, of course, is the ideal case in any piece we play; that we are skillful enough to be able to play it however we please despite the technical difficulties.Rarely is this the case, I find; on the contrary, we often hear all too many performances of Chopin Nocturnes and Ballades where the pianist is struggling. The mazurkas, however, allow for a low technical requirement with some truly wonderful music waiting. A true testimony to Chopin's mastery of small forms (though that doesn't take away from his larger compositions like the Ballades), and his melodic brilliance.