Chiang Mai has its own distinct cuisine, different from the food of Bangkok and the south. Northern Thai cooking is heavier, spicier, and more influenced by Burma and Yunnan than central Thai food. The key dishes to try: khao soi (curry broth with egg noodles), sai ua (herbed pork sausage), nam prik ong (spicy tomato-pork relish), and kaeng hang le (Burmese-style pork curry with pickled ginger).
Huen Phen on Ratchamankha Road is the most celebrated northern Thai restaurant in the city — lunch is cafeteria-style and quick, evenings are in a candlelit antique-filled room.
For khao soi specifically, Khao Soi Lam Duan Fah Ham across the river is the purists' choice. Mengrai Seafood near Tha Phae Gate handles fresh fish and shellfish at very reasonable prices.
The Night Bazaar area has dozens of restaurants; quality varies widely, and the tourist-facing places are often overpriced. Better to push a street or two back from the main drag.