As well as Hong Kong, i would have to say the best Dim Sum i had was in Malaysia, KL

It was in an upmarket hotel. It was the only DIm Sum buffet i have ever been to. You pay a fixed price, you order what you want and can order and eat as much as you want BUT you pay extra for anything not eaten. This ensures you don't over order. Man i left full as a staypuft marshmellow man ! And they had some really unique dim sum items