The best places for Dim Sum (IMHO) in D.C. are Lei Garden and Tony Cheng's in Chinatown (there's some evidence that Lei Garden is closed).
Funny story:
We were in Chinatown for Chinese New Year's on the 18th. Very crowded for the celebration. We had reservations at 3pm at Tony Cheng's, although they'd stopped selling Dim Sum by then. The ground floor of the restaurant is a Mongolian Barbecue, to which we've never been (although I like them) because we were always headed upstairs. It was, naturally, incredibly crowded, and if we hadn't had a reservation, we'd have bailed. My parents had arrived and were seated upstairs at the "seafood restaurant" (by which they mean there are huge fish tanks full of various seafood that you can order, including dungeoness crab, cooked in any style). We were at the bottom of the very steep 3'-wide staircase. On each step was a person standing, waiting for a seat in the restaurant. On the other half of the stairs was a stream of people either coming down or going up to use the only rest rooms in the building, located on the second floor. I stood at the bottom with my daughter and my sister-in-law, who was holding an infant car seat with her son inside, preparing to summon the nerve to struggle our way up the crowded stairway with said car seat. Suddenly, coming down the stairs, was the Mayor of D.C, Adrian Fenty! I recognized him immediately from the papers, and smiled hugely at him. He shook my hand and said, "How're you doing?" I said, "Great, thanks!" He then looked down at my nephew and said, "Aww, he's so cute!" before taking his leave. Wow! lol
Dinner was wonderful as always. I had a pork and asparagus dish which was wonderful and crunchy. Mr. Jennie had a spicy crispy beef dish, which was really spicy (to his delight). However, my mother had some and said she hadn't found it overly hot. Mr. Jennie said, "Well, you didn't eat any of the red peppers." Yikes! Does anyone eat those? I treat them like the whole cardamom pods and cloves in my Indian curries and put them on the side. Mr. Jennie says, "Oh, I eat those, too." lol

My brother, however, commented that the Orange Chicken he'd ordered for his 3-year-old son was actually spicier than his General Tso's chicken (and we did have the right ones identified. General Tso's had the red peppers in it.). However, the children all had plenty to eat, what with the fried rice and a PuPu Platter (which is always great fun for the kids). But one of the best things was the Peking Duck, which Tony Chengs always does well (we used to get it at his Szechuan restaurant in Baltimore years ago). Crispy duck skin available in easy-to-manage pieces, along with the juicy meat, and crispy crunchy scallions to put on your (nice and pliable) wrapper, with the wonderful plum sauce. They do this dish very well, and we rolled out of there well fed. One amusing and great surprise was for the baby in the car seat. My sister-in-law took her bottle of formula (which had been in a chilled bag) and asked the staff if it was possible to have it warmed. They brought it back to the table . . . in a teapot full of hot water! It was perfect! lol