• Omnicom, the second-largest advertising holding company by annual revenue, has become the most valuable business in a turbulent industry by share price and market cap. • Under John Wren, the company’s recent financial performance surpassed rivals on the strength of work for clients like AT&T, Apple, and PepsiCo despite a deep recession and a shift away from traditional revenue sources like broadcast ad campaigns. • Business Insider identified 31 of the most powerful people at the company, based on conversations with current and former Omnicom insiders, earnings reports, and other recent news.