Khao Sarn in Coolidge Corner on Harvard St in Brookline is excellent. Plenty of options, with and without meat, and plenty of heat as desired. I had pan fried noodles with ground chicken and it was very spicy at 2 chilis although it was offered at 3 chilis.
Everything tasted great, came in generous proportions, and looked appealing. The room and the service were also pleasant. There were many choices around $10 or under. Enjoy!