Choose any of the 48 Preludes & Fugues from The Well-Tempered Clavier (24 in Book I, 24 in Book II). You might wish to listen to a few before finding one that you like and have something interesting to say about.
You also might want to compare Angela Hewitt's performance on piano with Robert Levin's performance on organ or harpsichord. It could be much faster or slower, as Bach did not write tempo markings for these pieces.
Book I from Alexander Street Press: Nos. 1-12, Nos. 13-24
Book II via Columbia University Library or directly from Alexander Street Press: Nos. 1-12, Nos. 13-24
The link to the Liner Notes in the upper left-hand corner of each might help start you off. Be sure to cite them if you use them.