In which tennis “major" was Björn Borg the most successful?

Björn Borg (June 1956) was most successful in the French Open at Roland Garros with six wins. Björn Borg is a Swedish former world No. 1 tennis player widely considered to be one of the greatest in the history of the sport, who retired in 1983 aged 26. Between 1974 and 1981 he became the first man in the Open Era to win 11 Grand Slam singles titles (six at the French Open and five consecutive at Wimbledon).

The French Open is a major tennis tournament held over two weeks at the Stade Roland-Garros in Paris, France. It is the premier clay court tennis championship event in the world and the second of four annual Grand Slam tournaments, the other three being the Australian Open, Wimbledon and the US Open. The French Open is currently the only Grand Slam event held on clay, and it is the pinnacle of the clay court season. Because of the seven rounds needed for a championship, the slow-playing surface and the best-of-five-set men's singles matches (without a tiebreak in the final set), the event is widely considered to be the most physically demanding tennis tournament in the world.

His French titles were in 1974,1975, 1978, 1979, 1980 and 1981. He won Wimbledon five times consecutively between 1976 and 1980. He failed to win either the Australian Open or the US Open despite four final appearances in the latter. His winning streak at Wimbledon(41 matches) on grass was ended in 1981 in a titanic struggle with John McEnroe.