【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.