Owned and bred by Khalid Abdullah and trained by Henry Cecil, Frankel, who accumulated almost three million pounds ($4.82 million) in prize money, was awarded a final mark of 140 in the World Thoroughbred Rankings.
"An outstanding racehorse, it is fair to say in the circumstances that he constitutes a new benchmark for equine excellence on the racecourse," handicappers said in a statement released by the British Horseracing Authority.
The 140 figure puts Frankel ahead of Dancing Brave, who won eight out of 10 races in 1985 and 1986, including a scintillating Prix de l'Arc de Triomphe win.
Dancing Brave, also British-trained and owned by Abdullah, had been rated at 141 but this has been adjusted to 138 after a "historical recalibration of ratings" which took place because handicappers believe many horses in the 1970s and 1980s were given elevated figures.
"The primary purpose of the review was to acknowledge that handicapping methods and practices have evolved during the past 35 years and the resulting recalibration was designed to show how the ratings would look if they were being assessed on current levels," the statement added.
The international rankings system began in 1977.
Frankel, who won five Group One races last year, is about to embark on a lucrative career as a stallion, with his stud fee set at 125,000 pounds. ($1 = 0.6224 British pounds) (Writing by Justin Palmer, editing by Clare Fallon)