This is a city of roving sushi chefs, whose groupies follow them as they migrate from restaurant to restaurant. But Kiyoshi Hayakawa isn't going anywhere. After working at Sushi Ran and Tokyo GoGo, he has settled in at Koo, his 8-year-old restaurant, where he presides over a small, reservation-only sushi bar to the side of the main dining room. His classic nigiri and hand rolls are exquisite, the proportion of fish to rice ideal for tasting both, the rice molded so loosely that you sense every grain. Like the best sushi bars, the more you come back, the more you unlock the secrets of his sushi case, discovering fish you've never heard of or subspecies whose deliciousness few American diners have imagined. An omakase (chef's choice) dinner will cost you a cool hundred, but the procession of tastes puts it up there with a five-course dinner at Boulevard. By the time your uni arrives, sweet and quivering, you'll be begging the chef to let you buy him a beer.