Everyone says that sequels are never as good as the original. I agree that the generalization holds up pretty well. Two exceptions do come to mind.
1) The Lord of the Rings books. These were written as a sequel to the Hobbit. And they are better. The Hobbit is good, but the LOTR books are better.
2) The New Testament. Perhaps strictly speaking we shouldn't call it a sequel, since both the Old and New Testaments are actually anthologies of various books. But there is a sequel-like effect in the books that describe Jesus Christ and the New Covenant, a retelling and recasting of the story of God and his chosen people, Abraham and his descendants.
As a sequel, it attains something quite remarkable. In one sense it is a better story, but it also renews my curiosity to go back and read the original story as well.