There are dozens and dozens of Thai restaurants in the Sukhumvit area but many of them are, of necessity, more oriented towards foreign palates than Thai. Reflexions, in Terminal 21, is a place that manages to cater to both tastes. They have very traditionally prepared dishes like the shrimp with glass noodles above and the roast duck pictured below but they also have a wide variety of dishes that will appeal to those with more international tastes. 
Also, they have a good selection of regional Thai dishes that are well done. We sampled some mussaman curry a southern Thai dish, for example and were quite satisfied with the rich mix of flavors a that good mussaman should deliver. 
Some other dishes that we thought looked interesting on this large and varied menu include the deep fried sun-dried pork and the soft shelled crab with black pepper, the dried red chicken curry, the large selection of Thai salads — some spicy, some not — and the nine whole red snapper dishes among many others. Check out this eclectic-looking place — the decor is an interesting mix of knicknacks, artwork, some of which can be sold and interesting color arrangements —  for any number of good Thai dishes.
Reflexions, Terminal 21, 02-108-0961