I get the skinny on good Vietnamese from my father and he says Maxim is the best when you want Mi, the pork broth soup with egg noodles, wontons, barbeque pork, shrimp and the fixin's. He took me there and he's right. It is the best Mi soup in the Valley, if Dragonfly (in Mesa) doesn't give it some good competition.
I have to be honest and say that I haven't tried anything else, but I can tell you that SO MANY Chinese/Japanese restaurants of Maxim's caliber are really owned and run by Vietnamese. Deb K. is so right on this one--I can attest that the Chinese name and food is there to attract business. The real deal is the Vietnamese soup--the Mi. Those clever Vietnamese...
It's not very pretty, which others have stated, but if you can handle the sub-par cleanliness and decor in exchange for an amazing, albeit cheap bowl of soup, please do. It's definitely a Yelper's kind of place.