1.
I would use the above to ask for someone's advice on which they preferred of three or more choices, where the 'ranking' might go from least preferred to best. (this one, not at all, these two are ok, this one is very nice, but this one I like best.) In other words, good < better < best.
Most seems to me to connote a larger sample from which to choose. It is also asking more (to my way of thinking) for shades of interpretations instead of an authoritative opinion. Best limits the choices and the answer seems final.
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There are differences and similarities between Vietnamese and American cultures.
In America, two generations live in a home. In Vietnam, two, three or even four generations live under one roof. In America, old-aged parents live in nursing homes but in Vietnam they live with their children.
It is not polite to ask American about age, marriage and income. However, it is normal in Vietnam. Americans can greet anyone in the family first but in Vietnam, that's wrong. As a rule, all members have to greet the oldest one first.
Americans buy groceries once a week and keep them in a refrigerator. In Vietnam, inspite of having refrigerator, they still buy groceries every day. Maybe, they want fresh food, they consider that it is better for their health.
For American, Christmas and New Year holidays are the most important. But for Vietnamese, only New Year. One another feature, American children sleep in their own bedrooms, Vietnamese ones sleep with their parents so that they are cared well.
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3.
From my experience, I would say it depends a lot on your friends' taste. I realized that only Asian can stand the smell of fish sauce (and shrimp paste, especially). Whenever I cook Vietnamese dishes at home and have some leftover for lunch the next day, it's very likely that I'll have the whole kitchen area for myself (which I feel very sorry for :( ). My friends seems to enjoy Vietnamese food a lot, and because they said they want to taste authentic food, they had to endure the smell in the sauce we always use :P
Overall, they basically will go for anything when they're sure about wanting to try. So it's best to ask your friends first, to see what kind of food they can't eat, then explain to them a bit about what you're planning to cook. If it sounds good to them, go for it.
And my recommendation would be Hủ tiếu, cơm hến, cơm tấm or bún riêu, because you can find Phở or chả giò everywhere (even in places that doesn't even sell Vietnamese food