In Japanese ‘(A) の中（なか）で (B) が一番（いちばん） (C)’, is a regular expression that is used for showing that something is ‘the most (C)’, usually an adjective. This structure can be used with almost any adjective, whether it is positive or negative. Each part of this grammar point represents the following:
(A) - A group.
(B) - A single thing/person within that group.
(C) - An adjective, or general description of that thing/person.

• 食（た）べ物（もの）の中（なか）で寿司（すし）が一番（いちばん）好（す）き。
  Out of all foods, I like sushi best. (Sushi is the number 1 liked food)
• 授業（じゅぎょう）の中（なか）で理科（りか）が一番（いちばん）嫌（きら）いです。
  Out of all classes, I don’t like science the most. (Science is the number 1 hated class)

This expression is also frequently used with demonstratives such as この and その.

• この中（なか）でどれが一番（いちばん）好（す）き？
  Out of all of this, which one do you like best?
• その中（なか）ではクッキーが一番（いちばん）美味（おい）しい。
  Out of those, the cookie is the most delicious.

The construction ‘(A) の中（なか）で’ is regularly translated by itself as ‘among (A)’, and may be used in many different expressions. This will be covered in a separate grammar point.