I get quite a few emails from people planning to visit Bangkok asking me to recommend "the best places to eat" or where "the best Thai food" is. I'm honored that people would trust my opinion regarding an issue so profound, and thought the best way to approach this would be in a blog entry.
First of all, it's important to understand that with Thai food it's generally pretty hard to find a great all-around restaurant (although certainly some do exist); you're much more likely to find a place serving a few great dishes. As I've mentioned previously, the most rewarding Thai restaurants specialize in one style of cooking, or perhaps food from one particular region of Thailand. Keeping this in mind, it would be an immense undertaking to recommend individual restaurants. Instead, I'm going to mention the types of dining I think one should take part in when in Bangkok, and a few areas where small restaurants, street stalls and/or vendors are of a higher caliber than elsewhere.
For Thai newbies, I would strongly recommend beginning with a visit to a mall food court. They are clean and cheap, the menus are written in English, you have a wide range of choices, and actually, the food can be pretty good. My favorite food court is probably the one on the sixth floor of Mah Boon Khrong (also known as MBK). You'll find a huge variety of Thai food, everything from noodles to isaan--they even have a stall selling Thai-Muslim food such as khao mok kai. One of my favourite stalls is the one selling vegetarian food. There's generally a foodcourt at every mall, and in particular, upscale foodcourts seem to be springing up everywhere these days (such as the food court at Siam Paragon) but they're generally quite overpriced (by Thai standards) and mostly Chinese (I'm assuming you're looking for Thai food here). One peculiarity about Thai food courts: they don't accept cash. You'll need to find the cleverly hidden counter, where you'll have to stand in line to exchange your cash for coupons, or more recently, a swipe card. Then after you're done eating, you'll forget the coupons or swipe card with the remaining money in your pocket, and won't realize this until you get home. It's all part of the Thai food court experience.
At this point maybe you've found a dish or two that you like, and are somewhat more familiar with the flavours of Thai food. Now you are ready to eat somewhere "nice". Thus, I feel the natural next step is to eat at an upscale Thai restaurant. Be forewarned: upscale Thai restaurants are mostly mediocre, almost exclusively patronized by foreigners, and are going to be much more expensive than all other forms of Thai food put together. But they can also be very atmospheric and fun, and as most people try to include at least one on their trip anyway, I thought I would recommend the few I'm familiar with. My favourite upscale Thai place is probably La Na Thai, one of the restaurants in the lovely Face complex. I've eaten here twice, and both my Thai companions and I have enjoyed excellent Thai food each time. Other good upscale Thai include the tourist-ridden but good (as long as you avoid the buffet) Bussaracum and Flava. Lastly, if $ is not an issue, and you'd also like a view with your tom yam then I'd recommend the atmospheric riverside restaurants at the Oriental or the Peninsula.
Once you've downed a few plates of food court nosh and have consumed the requisite nice Thai meal, I reckon you're ready for the next step in Thai dining: a good food neighborhood. In my opinion, this is the highest level of Thai dining (you'll instantly realize just how average upscale Thai tends to be!). In good food neighborhoods there might be a few standout restaurants, but generally it's possible just to pick and choose. The restaurants are going to be simple, but the flavours strong. In this regard, I would recommend the area on and around Thanon Tanao in Ko Rattanakosin, one of Bangkok's oldest districts, and a place teeming with legendary Thai eats. Other good food neighborhoods include Tha Phra Chan (in particular the area around Tha Chaang in the evenings) and Thanon Phra Athit, both more or less located in the same area of Bangkok. I've also got a feeling that the Siam Square area might have some good eats, although on the surface it appears to be dominated by KFC and other chains. Investigation will ensue...
At this point, if you have followed my directions, you will have sampled a true cross section of Thai cuisine. It is only now that you are ready for the final step: Thai night market/street food. These affairs are only open at night, are not the cleanest restaurants you'll ever see, and they're in weird parts of town. But the food is often pretty good--almost equal to the experience. In this regard, I wholeheartedly endorse Chinatown at night. Simply walk down Thanon Yaowarat, try to avoid the annoying touts at touristy seafood restaurants, and pay attention as you reach the intersection Charoen Krung Soi 16. There you will find virtually every form of Chinese-influenced Thai street food. Another strong option is Sukhumvit Soi 38, where Chinese-ish food again dominates, but is a bit wider in scope, despite being a much smaller market.
There. That's my 2 bits. Anybody got anything else to add?