The top advertising agencies in terms of global clients and the brands that they serve are a trade secret. Industry professionals classify an agencies size by the total billings that they submit to their clients. Some of the larger ad agencies has total billings in the hundreds of millions of dollars. Interpublic Group is one of the largest in the world along with Dentsu headquarted in Tokyo, Japan. Some of the largest USA agencies are located in New York City.
In no particular order I will list some of the largest agencies that I can recall from my fellowship days working on Madison Avenue.
Young & Rubicam
Benton & Bowles
Ted Bates & Co.
Wunderman Worldwide, Inc. ( My home agency...130 offices in 55 countries as of June 2010)