The correct answer is Roger Federer of Switzerland. He has won 20 Grand Slam singles titles, more than any other male player in history.
Roy Emerson of Australia won 11 Grand Slam singles titles. He won six Australian Open titles, two French Open titles, two Wimbledon titles, and one US Open title.
Björn Borg of Sweden won 11 Grand Slam singles titles. He won five French Open titles and six Wimbledon titles.
Pete Sampras of the US won 14 Grand Slam singles titles. He won seven Wimbledon titles, five US Open titles, and two Australian Open titles.
Roger Federer of Switzerland has won 20 Grand Slam singles titles. He has won eight Wimbledon titles, six Australian Open titles, five US Open titles, and one French Open title. He is the only male player in history to have won at least five titles at each of the four Grand Slam tournaments. He is also the only male player in history to have won at least three consecutive titles at each of the four Grand Slam tournaments.