For us Americans who can’t make it out to Vietnam, I’ve found the best Pho in the US is in Orange County, CA and Boston’s Dorchester neighborhood!
Houston has excellent Vietnamese cuisine. I guess it's Southern Vietnamese style? (can't help but notice the ARVN veterans memorial on Bellaire Blvd.).
In the East, Falls Church in VA has excellent food. But I think the best would have to be Louisiana. No name places with a shrimp boat out back, on the coast, east of New Orleans. I'm told a lot of Vietnamese refugees were resettled there and went into the fishing industry, southern Louisiana and the Mississippi river delta being somewhat similar to Vietnam.
In San Francisco I love having pho in Chinatown. Golden Star Vietnamese Restaurant, Sai's Vietnamese, and Golden Flower are the ones I like the most.