If there's a fish sauce contest...
“The nuoc mam from Phu Quoc Island was the best of all, clear and with an astonishingly subtle taste. . .But the sauce in this restaurant is from the Philippines, very bad, not from Thailand, which at least is a pale second-best.” - from Love by Robert Olen Butler
(If you're a Filipino, look away quick, because I agree. I'm partial to Phu Quoc nuoc mam.)
When I went to Phan Tiet, the Vietnamese driver said that the nuoc mam from Phan Tiet is also good. I wouldn't know though since I didn't want to open the car to buy one while we were passing the nuoc mam area. It would have been tantamount to going inside a gas chamber. Or self-mutilation.