Seconding Nom Wah! The Doyers Street location feels like stepping back in time (it's been around since 1927), and the street itself is quite historic -- it was called the "Bloody Angle" in the 20s and 30s due to notorious gang fighting there, and was the site of the first Chinese-language theater in New York City. Not to mention, the place is beautifully quirky inside, and the food is delicious.
I love the soup dumplings at Shanghai Cafe on Mott Street, and Golden Unicorn for dim sum.