Khao Soy- a thick curry soup with fresh egg noodles, bite size pieces of chicken thigh and tender massamun-style beef, topped with deep fried crispy noodles and sliced shallots. It is unbelievable how tasty this is.
You can get this dish all over Chiang Mai, but I had it several times at Aroon Rai and it was consistently good. The Tuk Tuk drivers all know this place so you should be able to get there for 40-60 Thai Baht from most locations in town.