One place I often make the trip to is out at Si Yan, it's a noodle restaurant called Bamee Hooa Dtoh. What makes this one special is that they hand make their own noodles every day, around a 100 kilos of them, and sell out. They also prepare and roast all their own meats. Their stir fried noodles (bamee) with BBQ pork, crispy roast pork and geeo sap are superb (around 80 baht), deep fried spring roll pretty good too. The nam gra jiab is very good here too, very strong and not too sweet (15 baht). The restaurant has been going 50 years, it's open from around 9 IIRC until 7pm but frequently sell out before 2pm. It's near Dusit post office down Soi Si Yan 1. Included a link to Maps as it's a pig to find with search (mainly because Google does not list the English name (karaoke Thai) as per the Thai pronounced name): https://goo.gl/maps/v6U7Frsiqns
If you go there, come out of Si Yan 1 and turn right, walk up the main road about 150 yards on the right and there's a lady that sells the best hawm mok I've ever had. She again is very popular, often sold out by 1pm. They have pork and fish versions. Awesome. 20 baht ea.
I also really enjoy the kao sawy at Craft Cafe near BTS Phra Khanong. Very rich and creamy soup, and pretty spicy!