There are two types of all you can eat buffets. The Sizzler variety that features mass quantities of low quality friend carbohydrates and zoo quality breaded meat and the more upscale variety that feature high quality proteins and gourmet quality preperations.
The best current deal in Bangkok among the high quality gourmet quality banquets is at the Nai Lert hotel on Wireless Road opposite The Pent G-Club.
It cost 350++ THB, requires a reservation and features 5-star atmosphere and gourmet food.
The clientele is upscale and the Nai Lert is not friendly ground for bargirls and their dodgy roustabout boyfriends.
Here are pictures of the available food.