Som Tam Udon (ส้มตำอุดร) is one of the biggest, best and well-known Isaan restaurants in Chiang Mai. They make countless varieties of papaya salad and will let you customize it down to the ingredient, and the grilled chicken, fish and pork is endless. Unfortunately, it is also completely impenetrable if you can't read Thai. That doesn't mean you shouldn't go - you'll get a great meal of gai yang and som tam just by pointing, but if you want to take it to the next level you should invite a Thai friend.
Slightly pricier than a street stall, but a big meal won't run you more than 120 thb per person.