We can undoubtedly say that the 200-year-old Prado Museum is one of the leading art galleries in the world nowadays. Its collection includes about nine thousand paintings and seven hundred sculptures, among which are some of the masterpieces of Velázquez, Goya, Fra Angelico, Botticelli, Caravaggio, Hieronymus Bosch, Rubens, and hundreds of others.
It's very wise to visit the official website of the museum before your visit and plan your "march route" in case you don't want to lose a week inside the building. Which is also wise, by the way.