Mauritshuis
A 17th-century palace on the Plein holding over 250 Dutch and Flemish masterpieces, including Vermeer's Girl with a Pearl Earring, Rembrandt's Anatomy Lesson and Fabritius's The Goldfinch. It is a ten-minute walk from Den Haag Centraal.
Book a timeslot, and check the site first: the museum closes for maintenance in late summer.
Official site