Chiang Mai has one of the khao soi shops listed in the Michelin Guide — Khao Soi Mae Sai on Huay Kaew Road. The khao soi with nam ngiao broth is genuinely excellent. Both the beef and chicken versions are delicious, with fall-apart tender meat, really good. The curry itself is rich, and the crispy fried noodles are genuinely crispy (sometimes you find khao soi shops where the crispy noodles aren't actually crispy, and it just doesn't feel complete).
If you come at noon, expect to queue a bit. We'd recommend going in the afternoon — around 3:30 PM the place is still full of people, but you don't need to wait in line anymore. Open 8 AM–4 PM, closed Sundays. You'll need to find parking on your own around the area.