J.S. Bach is known for his difficult horn writing, most famously in the first Brandenburg Concerto and the Mass in B minor, but many of his cantatas also feature some extreme writing as well.  Historical research has shown that many of these parts may have in fact been played by trumpet players, but modern professional horn players can and do play these parts, and play them well!  One of the most impressive examples comes from the cantata Wär Gott nicht mit uns diese Zeit, BWV 14.  Check out this YouTube video featuring Hermann Baumann.  You can view the score on IMSLP here. The excerpt featured here begins on page 17, and the horn part is notated in B-flat alto.