What's your budget? Popular hotel buffets like Fairmont, Aquamarine, Carousel (Royal Plaza on Scotts) have good atmosphere and I feel have pretty good prices (see link below). Moreover, they usually have credit card discounts so that could take off a chunk of the price. My personal favorite is Carousel at Royal Plaza on Scotts. Good food and great ambience.
http://www.carouselbuffet.com.sg/prices/
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