The most … is X: adjective + nhất + là + noun

You know nhất after an adjective: Minh cao nhất. Minh is the tallest. To put the answer at the end, where it gets the weight, turn it round: the adjective and nhất first, then là and the answer. Học tiếng Việt, khó nhất là phát âm. The hardest part of learning Vietnamese is pronunciation.

It's a natural way to answer a "which is the most" question: Món nào ngon nhất? Ngon nhất là cá kho tộ. The best one is the clay-pot fish. A place or group often comes first to set the scene: Trong nhà em, cao nhất là ba. In my family, the tallest is Dad (in Hà Nội, bố). It works with feeling words too: Em thích nhất là đi biển. What I like most is going to the beach.

No word for "thing" or "one" is needed before nhất, though careful speech sometimes adds one: Điều khó nhất là …

Don't confuse it with nhất là after a comma, which means "especially": Em thích đọc sách, nhất là truyện. I like reading, especially novels. There, no adjective comes right before nhất.