MediaCom, which emerged in 40th place, was joined by five other agencies in the survey. TBWA\London moved up the ranking from 58 last year to 49 this year. It also came eighth in a poll assessing whether staff felt they were in their dream job. DDB London appeared as a new entry at 54, with staff praising its "buzzy, but family-feel atmosphere".
The media agency Carat, which has long claimed a place in the top 100, saw its position drop from 26 to 61, while the new entries MindShare and J. Walter Thompson came 76th and 95th respectively.
Abbott Mead Vickers BBDO, which has appeared for the past two years, did not feature in this year's results.
The list is compiled by measuring employee satisfaction across areas such as leadership, well-being and personal growth.
St Luke's came second in an additional poll, naming the top 50 small companies to work for. Staff cited their sense of involvement as an important reason for their happiness at the company.
The digital media agency i-level came 19th and the newspaper publisher Metro appeared at 50.
Despite its slide from 30th to 40th in this year's Top 100, MediaCom can boast that its result this year was boosted by top scores for its approachable management style. In the survey for confidence in the leadership of senior staff, MediaCom came in the top three with 81 per cent of staff saying that they had faith in the chief executive, Stephen Allan.
Allan said: "The Sunday Times 100 Best Companies survey is an excellent means for us to benchmark our work environment not only against our competitors, but against companies from all sectors of business. At MediaCom, we have learned that creating a great work environment and looking after our team is rewarded by better work and greater loyalty."