Have a look at Shostakovitch's 24 preludes and fugues; they are beautiful and very varied, from light-hearted and humorous to very deep and troubled. The C major is beautiful - the fugue itself uses only white keys and gets variety in the subject appearances by having the subject show up in all seven of the church modes. The E minor pair is fantastic, a beautiful, brooding prelude and an intense fugue. The A major fugue is written so that everything is always part of a triad - really clever and quick moving, too.
Depending on time, you could also consider Janacek's "In the Mist," a really beautiful, moving piece.