Ka Jok See (Phuket Town; tel. 07621-7903): A unique one, Ka Jok See -- smart decor, chic and elite, and very, very good Thai food. The kind of hush-hush upscale joint that attracts hotel GMs and mafia Dons.
May Kaidee (Bangkok; tel. 02629-4839): It's just tables in an alley-way, and you choose from a limited picture menu, writing your own order (and the service is hit or miss), but friendly May serves-up some of the best authentic Thai food going and it's all vegetarian to boot.
Golden Triangle Café (Chiang Rai; tel. 05371-1339): The Thai menu here tops the list -- almost a guidebook of Thai cuisine. Outside of Chiang Rai's busy market, this is the best dining going. A great meal, plus an education.
Khaomao-Khaofang Restaurant (Mae Sot; tel. 05553-2483): As much an oddity as it is a restaurant, the organic-themed decor of the upscale Khaomao-Khaofang would be wonderfully chic in Bangkok or Chiang Mai, but it comes off as somehow otherworldly in tiny Mae Sot, as do the portraits of Thailand's best and brightest who have visited. The place is out in the boonies on the Burmese border and the upscale design couldn't be more out of place, but the food is great and the atmosphere is laid-back, if a bit surreal.
Tatanka (Phuket; tel. 07632-4349): Just outside of the luxurious Laguna Beach Complex, Phuket's upscale community of resorts, Tatanka is the answer to dull hotel restaurants, a panoply of international tastes in a constantly evolving menu. Worth the trip from other beaches.