NEW YORK, June 5, 2024 /PRNewswire/ -- Omnicom (NYSE: OMC) has been named the Most Effective Holding Group in the 2023 Effie Index®, the 13th annual ranking of the companies behind the world's most effective marketing initiatives. This is the second time within the past three years that Omnicom has received the coveted global title.As part of Omnicom's strong performance, two of its agency networks placed in the top five of the Most Effective Agency Network category, with DDB Worldwide coming in second and ...