I've been going to Thai Long An weekly for four months now and each time has been great. It's a small family run restaurant with the best Thai in Phoenix.
I can say that it's authentic since my Thai friend had a long conversation with the owner and remarked how much it's like the food from his hometown. Pad Thai is how I judge Thai places and theirs is by far the best I've ever had. The pad see ew and e-San salad are some of my other favorites. Be sure to order a Thai iced tea when you go as well!
The staff is extremely friendly, get to know your name, and the service is pretty quick. Just make sure to enjoy really spicy food or ask for a 1 or 2 (out of 5) on the scale.