I've heard awesome things about Sal de la Terre, over by the Aquarium T stop. Never been there, but someday I want to go. There's also a fusion place in arlington called Mifune, which I've been to, which has the best sushi I've had on the east coast. If you're a sushi person and money is no object, I'd recommend going in there and talking to them about having the sushi chef decide what you eat.