Lao Laan Xang on Willy is a tiny little restaurant with unbelievable food. I've never had a bad meal there -- everything I've eaten has been tasty, fresh and authentic. My personal favorite is #33, which I believe is called Kua Curry Sai Phak (veggies and tofu in a coconut milk curry). The flavor is incredible. The service is very friendly and the servers are usually quite knowledgeable about the dishes if you are having trouble deciding.
The only bad thing about it -- and I mean only -- is that they serve cans of soda instead of unlimited refills from the fountain. If you have a Diet Coke addiction like I do, this could be a slight problem! Overall, though, it's among my very favorite restaurants in Madison.