Chief, principal, main, leading, foremost, capital mean first in importance or in standing.
Chief is applicable to a person that serves as the head of his class or group or to a thing that stands out as above all the rest of its class or kind in rank, importance, dignity, or worth; the term therefore usually implies the subordination of all others (chief justice of the supreme court).
Principal is applicable to whatever is the first in order of power or importance, and so is applied chiefly to a person to whom is given direction, control, or government of others or to a thing (or person thought of as a thing) that, because of its size, its position, or its intrinsic importance precedes all others of its class or kind.
Main is applicable to something (often a part, unit, or division of a large or extensive thing) that excels all the others of its class or kind in size, potency, or importance.
Leading, like principal, implies precedence, but it often distinctively implies, in addition, a capacity or fitness for drawing others, for guiding them, or for giving a particular quality or character to a movement.
Foremost differs from leading, which it otherwise closely resembles, in its stronger implication of being first in an advance or progressive movement; it is preferable for that reason whenever there is a suggestion of the person’s or thing’s having forged ahead to that position.
Capital is applicable to a thing that stands at the head of its class or kind because of its importance, its significance, its excellence, or its seriousness.