the very famous prelude and fugue in c# minor from the first book. the prelude is a lamento in more or less three parts; in a way it could be instructive to compare this with the c# minor prelude from book II, which is in a similar mood. although this is great music and writing, in book II there is definitely a higher freedom of counterpoint, and the lines are more amazingly independent. still, this has enough to convey a powerful mood.
the fugue is famous as being th eother fugue from the WTC besides b-fat minor book I in five parts, also the chiasmus shaped subject which has led so many commentators to relate this to christ carrying hsi cross to mount calvary. the entrance of the two counter subjects, the eighth note subject spiralling down, and the lament of the repeated notes in the quarter note subject are both dramatic. there is a stretto at the end between the main subject and the quarter note one, which builds a lot of tension.
all in all a triumph of bach's sublime imagination.
c# minor from book II:
https://www.pianostreet.com/smf/index.php?topic=46628.0(43)