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.
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.