For breakfast (7am - 10am, or when they run out) to have 'bánh mì ốp la'. Eggs, meat and served with bread. It's quite similar to Bò Né, but with about 4 or 5 different versions of pork instead of beef. I recommend getting a side order of paté too. I really enjoyed this place down an alley off the busy Cao Thắng. It cost me 48,000d for the standard bánh mì ốp la, the paté and the tea (which is included). Recommended breakfast!
"the first banh mi shop to reduce the size of their rolls to the now familiar 20 centimetres; they also substituted vegetables for expensive meats, to make the sandwiches more affordable for Saigon's working men and women"
— Apparently Hoà Mã was opened in 1958 (although the sign says 1960) and is said to be the origin of the modern banh mi
Update: Nov 2016 - I think the price has now gone up slightly but the food is exactly the same.