UK-based YouGov made its debut at 24 while France’s Mediametrie was 22nd. The rankings are based on 2008 revenue, which was up 3.9% to $18.8bn for the top 25 as a whole.

The absence of TNS from the list after its acquisition and integration into WPP’s Kantar Group caused some reshuffling throughout the table, notably pushing Kantar into second place behind Nielsen and forcing IMS Health down to third.

Ibope and ComScore were the fastest growing firms on the list, with revenue up 28% and 26% respectively. The remainder of the list had more modest single-digit growth, except for three agencies that saw revenues decline: Opinion Research (-0.3%), Harris Interactive (-10%) and Japan’s Video Research (-2.5%).

Of last year’s new faces, Cello Research & Consulting and BVA remained in the rankings, though OTX dropped out of the table.

The top 10 agencies are listed below. The complete top 25 rankings are published in the American Marketing Association’s Marketing News magazine and will appear online soon at www.marketingpower.com.

_{Notes: * Estimated by Top 25. ** For fiscal year ending March 2009. ( 1 ) Total revenues that include non-research activities for some companies are significantly higher. ( 2 ) Rate of growth from year to year has been adjusted so as not to include revenue gains or losses from acquisitions or divestitures.}