Other firms in the ‘very big’ category include Arm, David Lloyd Clubs, Dishoom, EDF in the UK - Customers, MBDA, Thomas Franks and Valor Hospitality UK & Europe.
The Leicestershire-based mutual was included after a nationwide survey found high employee satisfaction and a positive workplace culture.
The recognition highlighted the company’s efforts to maintain a supportive and high-performing culture, as well as its focus on staff satisfaction and training.
The recognition covered E.ON’s UK businesses, including E.ON Next, npower business solutions, and E.ON energy infrastructure solutions.
Gary Wright said: “flatfair’s recognition in this prestigious list underlines a culture that values openness, shared success and meaningful impact."
Abi Best said: “This recognition reinforces that we're on the right path, and we’re more motivated than ever to keep building a workplace where people truly thrive.”
EDF Customers scored among the highest for engagement in the 2,000+ employee group.
Reward featured within the list of businesses following analysis of 50,000 organisations across 26 sectors.