The final 200 pages of In Search of Lost Time (which count as basically a chapter in a book that big) are told like a book-length essay and grapple with the meaning of life itself. It is truly something life-changing.

Like another commenter has said, the final two chapters of Ulysses are serious contenders, as is the third chapter and Oxen of the Sun (?), where Joyce chronicles the birth of modern language through prose describing a birth. Ulysses, in general, is the best book ever written so really anything from it except the Wandering Rocks could count.

The final section of Tristram Shandy has the best joke in literature, and the chapter in The Recognitions that describes a party as a jungle should count for its amazing prose. Under the Volcano’s carousel chapter is great for its description of an alcoholic losing everything in his life before his eyes.