My favourite Thai restaurant would be
Salathip at the Shangri la Hotel. Nice riverside dining, good ambience and some great food.
The thing with Thai food is to eat well, you don't need the upmarket stuff as long as you stay away from sukhumvit. In my book there is hardly a decent thai restaurant along sukhumvit from Soi 1 to about Soi 39.
Another favourite is
Lan Na at Face Bar in Sukhumvit Soi 38. Wonderful ambience, not cheap, but well worth the money in my book. they also have an Indian Restaurant that is good.