The most: adjective + nhất

Put nhất after an adjective to say "the most" or "-est": rẻ nhất (cheapest), đẹp nhất (most beautiful), ngon nhất (tastiest, the best). There's no word for "the": Đây là cái rẻ nhất. This is the cheapest one.

To name the group, add trong (in) after nhất: Minh cao nhất trong lớp. Minh is the tallest in the class.

Nhất also follows feeling words such as thích (like) and sợ (be afraid of), after the thing: Tôi thích Đà Lạt nhất. Just as often you'll hear nhất là before the thing: Tôi thích nhất là Đà Lạt. Both mean I like Đà Lạt the most.

To ask, put nào (which) after the noun: Món nào ngon nhất? Which dish is the best? It's a useful question in any restaurant.

For "the least", Vietnamese usually takes the opposite word: "the least expensive" is simply rẻ nhất, the cheapest. You'll also meet hơn cả, "most of all": Tôi thích Đà Lạt hơn cả. But ít nhất means "at least", not "the least", and nhất can also mean "first, number one": thứ nhất (first), giải nhất (first prize).