The USA's oldest Chinatown, by virtue of massive 19th-century immigration from across the Pacific (which has helped give this city the most Asian cast of any in the country), San Fran's version vies with New York's as the country's most classic, as well as the largest outside Asia. Its covers 24 blocks of downtown, centered on Grant Avenue (with the classic "dragon gate", above) and Stockton Street (a bit less trafficked by tourists and more by locals, giving it the feel of a Hong Kong neighborhood). Ethnically most of said locals are Cantonese, and they and previous generations have created a fantastic array of dining and shopping options, as well as places to learn about Chinese-American history and cutlure such as the Chinese Cultural Center of San Francisco and the Chinese Historical Society of America Museum and Learning Center.  

Read more in my post 6 of the World's Choicest Chinatowns.