Saigon is my reference. But a lot of people like Quang's on Eat St in Minneapolis (I think it's meh, and Lu's down the street from them is better).
An interesting alternative - and an interesting restaurant! - is Ngon Bistro in St Paul, which is basically upscale foodie Vietnamese. They do a much more, well, serious banh mi than the more trad places, with better ingredients. Debatable whether it's an improvement, but it's worth trying, and Ngon Bistro in general is brilliant.
