Bamii, wheat noodles served with muu daeng, barbecued pork, is a dish you can find just about anywhere in Bangkok. The vast majority of stalls are leased out by a franchise called Chai Sii, and are mediocre at best. The noodles at these stalls tend to be quite pasty, and the barbecued pork is largely flavourless meat that appears to have been painted with a overly-sweet red sauce. I'm sure there must be some good bamii out there, but the only place that has reached my lofty standards thus far is Mangkorn Khao ('White Dragon'), way out in Chinatown.
I always order kiaow naam:
wontons in broth. The wontons are filled with ground pork that has been mixed with an ungodly amount of ground black pepper, garlic and coriander roots, and wrapped around a single shrimp. The broth is subtle, but fragrant, and includes a few bits of greens and deep-fried pork rinds. Eat this and I can guarantee you'll still be tasting the warm black pepper/garlic/coriander root flavour a good half-hour after leaving. It's a nice feeling.
I like to follow this with a bowl of bamii haeng muu daeng, 'dry' noodles with barbecued pork:
The noodles at Mangkorn Khao are slightly thinner than those you'll find elsewhere, and are toothsome and have a pleasant nutty flavour. The barbecued pork appears to have really been barbecued, and is slightly crispy and fatty, almost bacon-like. If pork is not your thing, you can also get crab meat, which I usually do at lesser stalls, but the pork here is so good I can't pass it up. If you order your noodles 'dry', the broth is served on the side, and I like to add just a couple tablespoons to loosen up the noodles.
Mangkorn Khao was mentioned previously here.
Corner of Thanon Yaowarat and Thanon Yaowaphanit (across the street from the corner of Th Yaowarat and Th Plaeng Naam)
Every day 7pm-12am (but they often run out as early as 10 or 11)