My favorite brands are Tra Chang (meaning "weighing scale") and Golden Boy. The latter is favored by my students for its endearing label – showing a baby boy sitting on a globe, cradling a bottle on the left arm with right thumb up. Both are excellent, adding a superb flavor to Thai dishes, but they are a bit harder to find than other brands, though well worth the effort of any search. Golden Boy is also available online from several online markets. Reasonably good are the King Crab and Anchovy brands. (Note: Kasma does not receive nor has she ever received any kind of remuneration from the manufacturers of these products. She recommends them because she thinks they are good products. Period.)
I do not personally recommend Three Crabs Brand, which several Asian cookbook authors recommend, mainly because it does not appear to be a naturally fermented fish sauce but is, rather, a flavor-enhanced, processed food product. According to the label, hydrolyzed wheat protein and fructose are among the ingredients – both are additives that have not been adequately time-tested for their potential long-term effects on health. Their inclusion suggests that the sauce is made through the process of hydrolysis, whereby a catalyst (sometimes from chemical sources) is added to hasten fermentation, allowing the company to produce large quantities of the product in shorter periods of time than would be required in natural fermentation.