I can say, without a doubt, that the best butter chicken I've had comes from Bollywood Masala in Fairfax, VA (10364 Fairfax Blvd, Fairfax, VA 22030). I've been all over the US/Canada/Middle East and haven't had anything that compares to this. It is authentic Punjabi food as you can get.

Things to order:
Butter Chicken
Kashmiri Chicken Korma
Vegetable Korma