Prelude and Fugue in e-flat Major, (St.Anne) BWV 552.
I love this fugue to death. We only get it on festivals as the postlude over at Lourdes, but it's wonderfully and intricately written. (Three themes, the third theme comes from part of a theme from the prelude, and during each theme, one of the other themes also plays. Listen carefully or you'll miss them.)