Look for "Viet Huong Brand." Its has 3 crabs on the label. Usually a lite pink color or it could be orange and red looking with an fish picture on it. You'll know what I mean if you see it. Most chinese markets have this stuff. Its the best nuoc mam.
I like the one in the middle, the one on the right can be a bit...bitter sometimes.
More nuoc mam info:
"If you want to stock your kitchen with the bare minimum for preparing Vietnamese food, start with good quality fish sauce (nuoc mam). How do you judge fish sauce? Look for a light amber color and the words nhi or thuong hang on the label.
These terms indicate that the condiment came from the first extraction of liquid from the fermented fish, and is of the highest quality. Grades of fish sauces are similar to that of olive oils. That is, extra virgin olive oil is more flavorful and costlier than virgin olive oil. This same rule applies to judging different fish sauces.
Also keep in mind that fish sauce is also used in Thai and Filipino cooking, where it tends to be saltier and heavier in flavor. So, even though Thailand produces most of the fish sauce sold in the U.S., you need to make sure that the condiment you’re buying is made in the ‘Vietnamese’ style. How to do this? Look for Vietnamese lettering alongside the Thai script. Sometimes, the terms Phu Quoc and Phan Thiet are included on the label to signal a connection with these two famous fish sauce-producing areas in Vietnam. Fish sauce from Vietnam now sometimes use "Hon Phu Quoc" or "Hon Phan Thiet" (see image on left) to indicate that their product came from islands in those regions; hon means islands, and communicates a more authentic Viet condiment (versus a Thai product, which may not be clued on on such linguistic subtleties).
One Pigeon brand offers different grades of fish sauce, using degrees on its labels to reflect how much the nuoc mam has been diluted with water before bottling. For example, "25° dam" means there's 25% fish sauce in the bottle. According to the producer, most fish sauces are 20%. One Pigeon comes in several intensities. The higher the degree, the more you pay.
If ca com is one of the ingredients, that's an indication that the end product was made from a high quality anchovy native to the waters surrounding the island of Phu Quoc. Though I like Viet Huong’s "Three Crab" brand for its consistently delicate flavor, aroma and color, I often try newer brands.
Some are saltier while others are sweeter. Let your palate be your guide. One Pigeon brand from Vietnam is pretty good too, but not yet widely distributed at the time of this writing; I found it only in Little Saigon in Southern California."
Depending on where you shop, there may be as many as a dozen brands to choose from. As price is often a reliable indicator of quality among Asian groceries, select a mid to high priced fish sauce (about $2-3 per bottle!)."