Suggestions:
Café Siam
40-42 Lyndhurst Terrace
Central
The food is tasty and reasonably priced. The Sashimi Prawns, with a tart and spicy sauce is a unique must try on the menu. The killer mekong magaritas will neutralize the spicy food.
Chili Club
1/F 88 Lockhart Road
Wan Chai
Serves some of the best Thai classics in Hong Kong. Try their beef or cuttlefish spicy salads, curries and pad thai - all at reasonable prices.
Thai Lemongrass
3/F California Tower
30-32 D'Aguilar Street
Lan Kwai Fong
Beautifully presented and delicious Thai food in a sophisticated space. Fixed price lunch $138 HKD. Frommer's Review
Rattan furniture, fake banana trees, and statues of Buddha set the mood in this upscale Thai restaurant in Lan Kwai Fong. The menu, drawing from regional cuisines from throughout Thailand and prepared by Thai chefs, includes such favorites as soft-shell crab with chile and garlic on fried basil leaves, steamed fish, and chicken in green curry. Unlike most Thai restaurants in Hong Kong, which often serve a watered-down version of Thai food, this one serves the real thing.
Taken from chinatravel.com and frommers.com