big gun

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun The most important or powerful person in an enterprise or field.
• noun Something of the highest value or influence.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun military A large artillery gun or one of the largest-caliber naval guns (as on a battleship).
• noun idiomatic, by extension, usually pluralized A person, group, thing, or course of action that is powerful, influential, or particularly effective.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun an important influential person