Singapore does have some excellent dim sum spots - my personal faves are (1) Imperial Treasure Super Peking Duck at the Paragon, (2) Crystal Jade Palace at Ngee Ann City, and (3) Taste Paradise at ION Orchard.
In Kuala Lumpur, my current personal favorite is Ming Ren Xuan at Avenue K:
You can also try these other ones in KL:
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Ming Room at Bangsar Shipping Centre, and its sister outlet, Tang Room at Starling Mall, Damansara Uptown.
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Elegant Inn also serves very good dim sum at lunch.
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La Sense at Damansara Kim.
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Li Yen at the Ritz-Carlton.
Surprisingly, for a city where its Chinese populace is mainly Cantonese, you do get some really awful, over-rated dim sum places - watch out for the Dolly Dim Sum chain.