At Din Tai Fung their dumplings are so good they earned a Michelin star. The service here is fast, and the decor is slightly on the upscale side—but you probably won't notice because you'll be focused on eating dumplings.
I've never really been impressed by xiaolongbao, or Chinese pork steamed dumplings. For one thing, I like mine fried. For another, I like dumplings with a lot going on, and xiaolongbao are simple. At most restaurants, simple means boring but not at Din Tai Fung.
As an added bonus, the restrooms here are very nice: real toilets, clean, with toilet paper. If you've lived in China as long as I have, you will understand what a rare treasure this restaurant really is.
If you're staying in the area of the Westin Hotel, this restaurant is a quick walk down 新源南路.