The National Gallery, London
The National Gallery in London is one of the best pound-for-pound museums in the world. Small, in terms of collection breadth (they don't have millions upon millions of items largely tucked away in storage) but unbelievable and unbeatable in terms of content. More "A works by A artists" than just about anywhere. When I had the opportunity to visit as a tourist, over from America for just 5 days in London, it blew me away. When I later had the opportunity to live in London for 2 years it regularly blew me away. Their special exhibitions are some of the best curated shows I've seen in "popular" or 'mainstream' museums. And they get amazing loans! Other museums evidently like to loan them their very best works, by their very best artists, resulting in very strong exhibitions.
Again it's a relatively small collection but it's all top quality work. Containing bona fide masterpieces by the likes of Da Vinci, Dürer, Michelangelo, Titian, Veronese, Holbein, Velazquez, Rubens, Van Dyck and Rembrandt... as well as an outstanding Impressionist collection and very strong British holdings... it is a truly remarkable and stunning museum.
The painting at left is a attributed to a young Michelangelo and is one of his few works on canvas.
I'll post more.