Not only one of the greatest pieces of music ever written, but one of the greatest achievements of any man in history. It's a spiritually powerful piece, emotionally powerful, structurally perfect.
Bach's "Chaconne" is also considered one of the most difficult violin pieces to master. Many try; few succeed. It's exhaustingly long -- 14 minutes -- and consists entirely of a single, succinct musical progression repeated in dozens of variations to create a dauntingly complex architecture of sound. Composed around 1720, on the eve of the European Enlightenment, it is said to be a celebration of the breadth of human possibility.