Guay Tiaow Lot (ก๋วยเตี๋ยวหลอด) is a noodle truck that only does two things, but they're both incredible and extremely hard to find. The noodles are topped with tofu, steamed pork, shiitake mushrooms, squid and a quail egg with a powerful broth that's so awesome you'll find it hard to eat at a regular noodle place again. The fresh spring rolls have real crab in them, topped with a sweet sauce and served with yellow mustard, which is totally weird but somehow works. The only other place that does anything similar is Rot Yiam Beef Noodles, which isn't quite as delicious but still worth going to for the rest of their food.
Unfortunately, to score some noodles and spring rolls you'll have to brave unfriendly staff and short, unpredictable opening hours. Supposedly they keep going till 3pm, but I've seen them run out of everything before 1:30. They're also prone to being closed in the middle of the week, but if they are then the aforementioned beef noodle restaraunt is a good alternative. Worth noting is that there's a different food truck called "Enough Steak" that parks in the exact same spot after dark. It's... not recommended.