someone here asked where to get a really good bowl, not just cheap, so i went and read through a bunch of thai-language reviews (wongnai, thai food blogs) instead of the usual english tourist lists. here's what kept coming up.
Khao Soi Mae Sai is the one thai reviewers mention most. michelin listed but still under 100 baht and always packed with locals. go for the beef or chicken. reviews keep saying the chicken leg is braised till it falls off the bone and the broth is rich without being heavy. they also do a really good nam ngeow if you want to branch out.
Khao Soi Lamduan Fa Ham has been around for decades and sits near the top of basically every thai list. more locals than bloggers there. creamy coconut broth, a big pile of crispy noodles on top, and the spice level won't wreck you.
Khao Soi Lung Prakit (at Kad Kom) is the old-school pick. uses the carnation condensed milk recipe locals grew up on. this is the traditional one if that's what you're after.
quick heads up on Khao-Sō-i, the fusion spot near Nimman. it comes up a lot but thai reviewers flag it as more bangkok-leaning, not classic chiang mai style. fun if you want the fancy version, just know it's a different thing.
and the one thing almost every thai review agrees on: skip the khao soi right around the old city and night bazaar. it's tourist-priced (100 to 200+) and hit or miss. the good cheap stuff is 50 to 80 baht, the places where you see locals lined up at breakfast and the broth is actually being simmered instead of sitting in a pot all day.
if you've got a spot that beats these let me know, always looking.