In the city, Hutong is good.
Camy's is better
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Bit further out, there's Auntie's
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In Glenny there's JG's
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They're all good. They're cheap, they look nasty, but the food is awesome. Don't go there if you're trying to impress people with decor or expect quality service though. Golden rule with dumpling houses is that the ruder the staff, the better the food.