The city's oldest teahouse, Yank Sing began in Chinatown in the late 1950s but moved to the Financial District in the 1980s. The kitchen prepares 100 varieties of dim sum on a rotating basis, serving some 60 varieties daily. The Spear Street location, near one end of the Rincon Center atrium, is big and comfortable; the older, Stevenson Street site is smaller, a cozier refuge for neighborhood office workers who fuel up on steamed buns and parchment chicken at lunchtime. Take-out counters in both establishments make a meal on the run a delicious compromise. Reservations essential. Other location: One Rincon Center, 101 Spear St., PHONE: 415/957-9300; Reservations essential;