Every Hanoi local has a favourite pho stall, and the arguments over which one is best have been running for decades.
Pho in Hanoi is a plastic-stool, low-table affair, usually served from a single pot by a single family for a single meal — some stalls do breakfast pho only and close by 10am. The broth here is cleaner and less sweet than the versions you may know abroad, built on charred ginger and onion, simmered with beef bones for the better part of a day.
Do the crawl properly: one bowl of pho bo (beef) near Dong Xuan Market in the morning, a plate of bun cha — grilled pork patties in a sweet-sour broth, invented in Hanoi and still best there — for lunch, and egg coffee in the afternoon at a café that's been pouring it since the 1940s. Between stops, walk off the sitting; the Old Quarter's 36 streets were each once dedicated to a single trade and the names still tell you which.
A practical tip: watch which stall the motorbike taxi drivers are eating at. They know the broth better than any guidebook, and they don't have time to waste on a mediocre bowl.