Note: This is a list from 2012 and is focused more on freestyle swimmers and multiple Olympic champions. I do not plan to update it.
1. Mark Spitz, born 1950
Mark Andrew Spitz (nicknamed Mark The Shark) is a retired American swimmer. He won seven gold medals at the 1972 Munich Olympic Games, an achievement surpassed only by Michael Phelps who won eight golds at the 2008 Olympics. Between 1968 and 1972, Spitz won nine Olympic golds plus a silver and a bronze, five Pan American golds, 31 US Amateur Athletic Union titles and eight US National Collegiate Athletic Association titles. During those years, he set 33 world records. He was named World Swimmer of the Year in 1969, 1971 and 1972.
2. Michael Phelps, born 1985
Michael Fred Phelps II (born June 30, 1985) is an American competition swimmer and the most decorated Olympian of all time, with a total of 22 medals. Phelps also holds the all-time records for Olympic gold medals (18, double the second highest record holders), Olympic gold medals in individual events (11), and Olympic medals in individual events for a male (13). In winning eight gold medals at the 2008 Beijing Games, Phelps took the record for the most first-place finishes at any single Olympic Games. Five of those victories were in individual events, tying the single Games record. In the 2012 Summer Olympics in London, Phelps won four golds and two silver medals, making him the most successful athlete of the Games for the third Olympics in a row.
Phelps is the long course world recordholder in the 100-meter butterfly, 200-meter butterfly and 400-meter individual medley as well as the former long course world recordholder in the 200-meter freestyle and 200-meter individual medley. He has won a total of 77 medals in major international long-course competition, totalling 61 gold, 13 silver, and 3 bronze spanning the Olympics, the World, and the Pan Pacific Championships.
3. Ian Thorpe, born 1982
Ian James Thorpe (nicknamed the Thorpedo and Thorpey) is a retired Australian freestyle swimmer. He has won five Olympic gold medals, the most won by any Australian, and, at the 2001 World Aquatics Championships, he became the first person to win six gold medals in one World Championship. In total, Thorpe has won eleven World Championship golds, the second-highest number of any swimmer. Thorpe was the first person to have been named Swimming World Swimmers of the Year four times, and was the Australian swimmer of the year from 1999 to 2003. His athletic achievements made him one of Australia’s most popular athletes, and he was recognised as the Young Australian of the Year in 2000.
4. Aleksandr Popov, born 1971
Aleksandr Vladimirovich Popov is a Russian former Olympic gold-winning swimmer, widely regarded as one of the greatest sprint freestyle swimmers of all time.
5. Pieter van den Hoogenband, born 1978
Pieter Cornelis Ruud Martijn van den Hoogenband (nicknamed The Flying Dutchman and Hoogie) is a Dutch former swimmer and a triple Olympic champion.
6. Johnny Weissmuller, born 1904 – died 1984
Johann Peter Weißmüller was an Austro-Hungarian-born American swimmer and actor. Weissmuller was one of the world’s best swimmers in the 1920s, winning five Olympic gold medals and one bronze medal. He won fifty-two US National Championships and set sixty-seven world records. After his swimming career, he became the sixth actor to portray Tarzan in films, a role he played in twelve motion pictures. Dozens of other actors have also played Tarzan, but Weissmuller is by far the best known. His character’s distinctive, ululating Tarzan yell is still often used in films.
7. Grant Hackett, born 1980
Grant Hackett is an Australian former swimmer most famous for winning the men’s 1500 metres freestyle race at both the 2000 Summer Olympics in Sydney and the 2004 Summer Olympics in Athens. This achievement has led him to be regarded as one of the greatest distance swimmers in history. He also collected a gold medal in Sydney for swimming in the heats of the 4×200 m freestyle relay. He is well regarded for his versatility, being the world record holder in the 1500 m and formerly in the 800 m freestyle, and 2nd and 4th in the 400 m and 200 m freestyle respectively. He has dominated the 1500 m event in the past decade, being undefeated in the event in finals from 1996 until the 2007 World Aquatics Championships. His four World Championship gold medals in the event make him the only swimmer to have won a world title in one event four times, and in total, he has won 10 World Championship gold medals.
8. Krisztina Egerszegi, born 1974
Krisztina Egerszegi is a Hungarian former world record holding swimmer and one of the greatest Hungarian Olympic champions of the modern era. She is a three time Olympian (1988, 1992 and 1996) and five time Olympic champion; and one of two individuals to have ever won the same swimming event at 3 consecutive Summer Olympics.
9. Debbie Meyer, born 1952
Deborah Elizabeth Meyer is a former American swimmer who won the 200, 400, and 800 m swimming events at the 1968 Summer Olympics in Mexico City. While still a 16-year old student at Rio Americano High School in Sacramento, California, she became the first swimmer to win three individual gold medals in one Olympics.
10. Kristin Otto, born 1966
Kristin Otto is a German Olympic swimming champion. She is most famous for being the first woman to win six gold medals at the 1988 Seoul Olympic games.
Matt Biondi, born 1965
Matthew Nicholas Biondi competed in the Summer Olympic Games in 1984, 1988 and 1992, winning a total of eleven medals (eight gold, two silver and one bronze). During his career, he set seven individual world records (three in the 50-meter freestyle and four in the 100-meter freestyle). At the 1988 Olympic Games in Seoul, Biondi won five gold medals, setting world records in the 50-meter freestyle and three relay events.
Ian Murray Rose, born 1939 – died 2012
Iain Murray Rose was an Australian swimmer, actor, sports commentator and marketing executive. He was a six-time Olympic medalist (four gold, one silver, one bronze), and at one time held the world records in the 400-metre, 800-metre, and 1500-metre freestyle (long course). He made his Olympic debut at the 1956 Summer Olympics as a 17-year-old and won three Olympic medals, all gold. Four years later, as a 21-year-old, he won three Olympic medals (one gold, one silver, one bronze) at the 1960 Summer Olympics.
Note: Thank you for your suggestions, they are welcome and will be included in the “Special mentions” category below.