Swiss player Roger Federer has won the most men's singles Grand Slam titles in tennis history at 20: six Australian Open, one French Open, eight Wimbledon and five US Open titles. Federer is still playing.
Australian Margaret Court has won the most women's singles Grand Slam titles in tennis history at 24: 11 Australian Open, five French Open, three Wimbledon and five US Open titles. Court retired in 1977.
American Serena Williams has 23 women's singles Grand Slam titles: seven Australian Open, three French Open, seven Wimbledon and six US Open titles. Williams is still playing.