Best. Art museum. Ever. That’s my opinion, but let me quickly say that world-renowned critics also consider the 78-year-old National Gallery of Art to be among the best museums in the world. Its base collection of more than 130,000 paintings, drawings, prints, photographs, sculpture, decorative arts, and furniture trace the development of Western art from the Middle Ages to the present in a manner that’s both informative and rapturously beautiful.
Now let me tell you why this is my favorite art museum, even one of my favorite places in Washington. I love the many ways the Gallery’s design and programs make the artworks and the museum itself accessible to the ordinary visitor. Architect John Russell Pope (of Jefferson Memorial fame) modeled his design of the original West Building after the Pantheon in Rome, anchoring the main floor’s interior with a domed rotunda, and then centered a colonnaded fountain beneath the dome. The overall feeling is of spaciousness and grace, especially when the huge fountain is encircled with flowers, as it often is. Extending east and west of this nexus are long and wide, light-filled, high-ceilinged halls, off which the individual paintings galleries lie, nearly 100 in all, leading eventually to lovely garden courts and more places to sit.
One hundred galleries? Yes, but the 1,000-some paintings are arranged in easy-to-understand order, in separate rooms by age and nationality: 13th-century Italian to 18th-century Italian, Spanish, and French artists on the west side; 18th- and 19th-century Spanish, French, British, and American masters on the east side. You may recognize some names: Leonardo da Vinci (whose ethereal painting, Ginevra de’ Benci, which hangs here, is the only da Vinci painting on public view in the Americas), Rubens, Raphael, Cassatt, El Greco, Brueghel, Poussin, Vermeer, van Dyck, Gilbert Stuart, Winslow Homer, Turner, and so on.
Down the sweep of marble stairway to the ground floor lie the West Building’s remaining galleries. The light-filled, vaulted-ceilinged sculpture galleries include standouts by Bernini, Rodin, Degas, and Honoré Daumier, whose 36 small, bronze busts of French government administrators are highly amusing caricatures. Other galleries display decorative arts, prints and drawings, photographs, even Chinese porcelain.
Across the street from the West Building is a Sculpture Garden that features 20 sculptures created by an international roster of artists in the last few decades, as well as a stunning Chagall mosaic.
The I. M. Pei–designed East Building showcases modern and contemporary art in galleries that lie off a dazzling atrium and includes three skylit towers and an outdoor sculpture terrace with a grand view of the city. In the East Building another world opens up, as graceful as the West Building, but here it’s angular, airy, and capricious. An immense and colorful Calder mobile floats overhead, but where are the galleries? You’re meant to wander, but you might miss something without a strategy. So here goes:
After arriving via the underground walkway from the West Building, find the elevator that will take you to the rooftop and its two towers. Tower 1’s two-gallery space presents a study in contrasts, one gallery devoted to an array of mesmerizing colorblock Rothkos, the other displaying Barnett Newman’s abstract, muted depictions of the Stations of the Cross. Tower 2 holds an entire roomful of Calders, many small and toylike, some swaying mobiles suspended from skylights, all colorful whimsies. 
Take the elevator inside the Calder exhibit to the “Upper Level,” one floor below, and head rightward out of the elevator, through the galleries on minimalist art and across the atrium to discover the softly lit room hung with 10 Matisse cutouts, decorative designs on paper that are mounted on canvas. Most attention-getting is “Large Decoration with Masks,” a wall-covering paper mural of brightly colored rosettes. (This gallery’s hours are limited, so make sure you visit before 2pm Mon–Sat, and before 3pm Sun.) The third tower is for temporary exhibitions; whether you go will depend on what’s on.
Okay, now you’re good to go explore on your own, back through the galleries of minimalist art, or on to pop art, photography, Picasso, American art from the first half of the 20th century, and French paintings from the last half of the 19th century and first half of the 20th. 
If you make your way back to the West Building and exit onto 7th Street, you are directly across from the Sculpture Garden. Go! Positioned throughout its lushly landscaped 6 acres you’ll find a stalking Spider by Louise Bourgeois, a shiny stainless steel and concrete tree called Graft, by Roxy Paine, and 17 other modern sculptures. In the northwest corner is a delightful, large (10x 17 ft.) glass and stone mosaic by Marc Chagall.
At the center of the Sculpture Garden is an expansive pool, which turns into an ice rink in winter. The garden is famous for its summer Friday Jazz in the Garden series of concerts, which are free and draw a crowd.
The National Gallery also mounts killer special exhibits and offers a robust year-round schedule of films, tours, and talks, monthly after-hours events from October to April, and Sunday concert series (now in its 77th year)—all free, let me emphasize.
Not free, but recommendable, are five dining options, the best of which are the Garden Café (its menu sometimes is tied to the theme of a current exhibit) and the Sculpture Garden’s Pavilion Café.
We have Andrew W. Mellon to thank for the museum. The financier/philanthropist, who served as ambassador to England from 1932 to 1933, was so inspired by London’s National Gallery that he decided to give such a gift to his own country. The West Building opened in Washington, D.C., in 1941, the East Building in 1978, and the Sculpture Garden in 1999.