One of the greatest archaeology museums in the world, holding two collections of extraordinary depth: the Farnese collection of ancient Roman sculpture (the Farnese Bull and Farnese Hercules are the set-pieces) and the Pompeii collection — the finest mosaics, bronzes, and frescoes rescued from the buried city, including the Alexander Mosaic (80 BC) and the contents of the Secret Cabinet (erotic art from Pompeii, now accessible).
Allow a full morning. The Pompeii rooms alone justify the trip; the Secret Cabinet requires a separate (free) timed entry. This is the essential context for any visit to Pompeii or Herculaneum.