Carat, which is the Dentsu Aegis Network agency, held on to the top spot in February 2014 and 2015. However it dropped one place in May 2015 in Recma’s Network Diagnostics report.
MEC, the WPP-owned shop, took the third position in the top global agencies table. It means that OMD has moved from third place to fourth place, knocking ZenithOptimedia to fifth place.
Vizeum has jumped four places to ninth. Maxus, which was named the world's fastest growing media agency network over the past five years in last year’s Recma report, has fallen from tenth place to 13th.
MediaCom said its leadership follows recent new business "success" including snapping up the Mars global media planning business.
In April, Tesco moved its £90m media planning and buying account to MediaCom, ending its 20-year relationship with Initiative.
Stephen Allan, the worldwide chairman and chief executive at MediaCom, said: "We’ve worked incredibly hard over the last few years to consistently build the global scale and expertise it takes to commit 100 per cent to all our clients while still winning new business."
The UK scores were calculated based on qualititative scores for Vitality and Structure, with MediaCom scoring 12 and 11 respectively. Carat scored 11 for Vitality and 10 for Structure.
Top 10 agencies globally (May 2015)
- Havas Media
- Starcom Media Vest Group
Top 10 agencies in the UK (May 2015)
- Starcom Mediavest Group
- MG OMD
- OMD UK
The next diagnostics report will be published in October.