Unfortunately, LA's Chinatown is somewhat dead. And while there are good restaurants (Ocean Seafood and Empress Pavillion), you have to head east to Monterey Park for the prime Chinese food.
NBC Seafood has, in my opinion, the best dim sum in town. The dishes taste fresher, and in spite of the large numbers of tables they turn, the food does not taste mass-produced, like at other restaurants.
Expect long lines on the weekends and avoid days like Mother's Day. If you get there by 10:30am on a normal weekend, the wait should be short.
NBC does the standards well: Siu Mai (wonton-wrapped steamed dumplings), Har Gow (shrimp dumplings in a rice wrapper), and Cha Siu Bao (steamed barbecue pork-filled dumplings).
But don't limit yourself. NBC's selection is dizzying. Inspect the contents of every cart that comes your way and hope to God you don't get seated in a lousy cart traffic area.