(This might have been posted twice)
I recently went to Boon Chu (83-18 Broadway, Elmhurst) for the first time, and (only) had tom yum koong soup, which I thought was very good.
It was very spicy, they didn't skimp on the shrimp (size or number), and the portion was large enough to be a small meal (larger than a pre-entree appetizer).
It was an authentic Thai dish at a minimum (but I have no opinion on whether it was as good as the same at other authentic NYC Thai places, since I haven't had the soup at the others).
Full disclosure: a Thai friend I met in Surin Thailand didn't want to eat there (and of course she wouldn't tell me why, when I made the suggestion).
The "restaurant" is in a former Chinese takeout place, so ambience is zero. Dishes are cheap (appetizers and entrees same price, generally $5.50 or $6.00)