I'll also agree with the Sripraphi rec.
Another very very good thai place is Jai Ya, on 3rd and 28th. Kittichai in Soho is a scene, and there's a lot more going on there than just the food. I've heard it's good, albeit expensive. But I haven't been there.
I've never been to Chelsea Thai, so I may put that on my list.
There has been a good amount of debate on this topic before: http://www.chowhound.com/topics/36982....
Sripraphai in Queens is obviously going to win any contest in the city when it comes to Thai food, but if you're looking to stay in Manhattan, I find Chelsea Thai is probably the most authentic, with the caveat that they can be somewhat inconsistent.