I've had far better dim sum in Hong Kong and Vancouver, but in Taiwan, the Hong Kong based chain Sweet Dynasty takes the top prize in terms of pricing, availability, convenience, and operating hours. The ingredients are fresh, and always piping hot when ordered, the restaurant stays open later than most places (10:30) it's located in the heart of Taipei (with other locations around Taiwan), and the price range is decent (80-100 NT per dim sum).
If you're not in the mood to eat dim sum, they also offer other kinds of typical Cantonese fare such as wonton noodle soup, fried rice dishes, sautéed dishes like Ging Do (sweet and sour) pork, and heavenly desserts like Dou Fu Fa (sweet tofu) and Mango pudding.
I usually make a trip to Sweet Dynasty at least 3 times a month!