Unless you're Robert De Niro (he's a partner and eats here regularly), you need to make reservations a month in advance at New York's most famous Japanese restaurant. A curved wall of river-worn black pebbles, a hand-painted beech floor, bare wood tables, and sculptural birch trees set the stage for Nobu Matsuhisa's dramatic food. The vast menu can take you down two paths. One road is the way to classic Japanese sushi and sashimi, among the best in town. Another leads you to contemporary dishes, such as the paper-thin hamachi spiced up with jalapeño or sea bass topped with black truffle slivers. It's easiest to put yourself in the hands of the chef by ordering the omakase omikase -- specify how much you want to spend (the minimum is $80 per person) and the kitchen does the rest. Can't get reservations? Try your luck at Next Door Nobu, where diners can enjoy a slightly less-expensive menu on a first-come, first-served basis. Reservations essential. Subway: 1, 9 to Franklin St. Other location: 105 Hudson St., at Franklin St., PHONE: 212/334-4445; Subway: 1, 9 to Franklin St.