First let me state that i have lived in Thailand for more than three years. Yupha's Kitchen is the best Thai food in Phoenix.
Thai food is my favorite cuisine. The French invaded Thailand and held it as a colony for may decades. During that time the French cooking techniques were merged with the traditional Asian ingredients and techniques. The result is modern Thai food, the greatest food today.
So with regard to rating a specific restaurant there are three components. The authenticity of the food, the quality of the food, and the quality of the service.
Yupha's Kitchen is a winner on all three. The owner (always present) definitely knows how to cook authentic Thai recipes. The mix of fish sauce and lime is perfect, the mix of protein and spice is perfect, and the mix of textures is perfect. Secondly the ingredients that are served are top notch. I have eaten at many Thai restaurants that really do get the spice mix part but want to serve less than top grade proteins. At Yupha's you will get very fresh chicken or beef or pork. Thirdly the service at Yupha's is amazing.
As a world traveler, I will say not only is Yapha's the best Thai food in Phoenix, it is one of the best in the west period.
Wes