In trying to determine who the greatest athlete of all-time is, one must first define greatness. Webster’s Dictionary defines great as “remarkably skilled” but that is not the only definition that great can take on. Greatness could be defined as returning from a life-threatening cancer diagnosis, like Lance Armstrong. Or greatness could be rewriting the record books since your career began, like Tiger Woods. Another sign of greatness is completely revolutionizing your sport in the way that Wayne Gretzky, Michael Jordan, Muhammad Ali and Roger Federer have done in their time. Digging deeper into each athlete’s accomplishments and their affects on the sport they played can better determine who is in fact the greatest of all time.

Lance Armstrong was a cycling protégé at age 13 when he won the Iron Kids Triathlon and five years later he qualified for the junior national championships in Moscow in 1989. In 1991, Armstrong won the U.S. National Amateur Championships and officially became a professional rider. He became the number one rider in the world and was given a spot on the Olympic team in 1996, but later that year his life changed drastically. While riding in ’96 he was forced off of his bike in enormous amounts of pain, and in October of that year his doctor had an answer for his pain, cancer. Armstrong had testicular cancer that had spread to both his brain and abdomen and his doctors gave him less than 50% survival rate and he began chemotherapy that October.

Remarkably Armstrong survived and when his cancer went into remission and he had his last round of chemotherapy in December of 1996 and returned to the bike in January of 1998. Beginning in 1999, Armstrong won seven consecutive Tour de France titles before retiring in 2005. This year, Armstrong came out of retirement to become the oldest person to be on the podium after finishing in third. Greatness for Armstrong can be described without making a single mention of his Tour de France victories. He returned from the brink of death to defeat cancer and by returning to the bike, brought cancer treatment and survivors to the forefront. Armstrong single-handedly brought cycling to the world’s attention by giving it a face that they can be proud of. Armstrong may have seven Tour de France victories, but his victory over cancer was his biggest of all.

Eldrick “Tiger” Woods will one day go down as the greatest golfer of all-time, but how does he compare to the other athletes on this list? Woods, born in 1975, began golfing at the age of two and in 1978 in an appearance on The Mike Douglas Show had a putting contest against comedian Bob Hope. In 1994 Woods became the youngest winner of the U.S. Amateur Champion, a record that stood until 2008. Woods officially became a professional in August of 1996 and signed $60 million in endorsement deals with Nike and Titleist. Since becoming a professional, Tiger has won 14 Major Championships, ranking him second all-time to Jack Nicklaus’s 18, including winning the “Tiger Slam” as the winner of all four majors from 2000-2001. Woods also has 71 PGA Tour victories and 37 victories on the European Tour, both rank him third all time.

Woods came back from a knee injury this year at the WGC-Accenture Match Play Championship in what the AP called “one of the most anticipated returns in sports.” He has revolutionized the sport of golf by becoming the face of the game. Due to his success, some courses within the PGA Tour rotation began to move the tees back to add yardage to the course. This was done to try and slow the long hitters, like Woods, and was known as “Tiger-proofing.” Woods himself welcomed the change by saying that adding distance to the course does not affect his ability to win tournaments.

Wayne Gretzky holds 60 NHL records combined from the regular season, postseason, and All Star games. Among those records he holds the NHL record for goals, assists and points in both the regular season and the playoffs in a season and for his career. He is the only player in the NHL to reach 2,000 regular season points and 3,000 points in the playoffs and regular season combined. In his time in the NHL Gretzky won four Stanley Cups, all with the Edmonton Oilers. He never won an Olympic Gold medal but he did win three gold medals playing in the Canada Cup. Gretzky also won nine Hart Trophies, the NHL’s MVP trophy, and has the most MVP trophies of any player in North American professional sports.

Gretzky has been named “the greatest player of all time,” according to Total Hockey: The Official Encyclopedia of the NHL and by numerous players, coaches, writers and fans. He was credited with the expansion of the NHL to the west coast after his trade to the Los Angeles Kings. His jersey number 99 has been retired by all teams throughout the NHL and he was the last player inducted into the Hockey Hall of Fame to bypass the three-year waiting period. He was known for setting up behind the goal, an area that was named “Gretzky’s office” due to his skills there. In his last game two large 99’s were painted on the ice behind the goal in honor of his play there.

In one of the most recognized high school sports stories off all-time, Michael Jordan was passed over for his varsity high school team because he was deemed to be too short. Being cut from the team, and fueled by his legendary competitiveness, Jordan came back the following year four inches taller to make the team, and the rest is history. Jordan won six NBA Championships, with two “three-peat” performances, and was named the Finals MVP of all six championships. He was a five-time regular season MVP selection and won 10 NBA scoring titles. Jordan was also an 11-time All-NBA selection and was named to the All-NBA Defensive team nine times including winning the Defensive Player of the Year in 1988. In 1984 Jordan won the Adolph Rupp trophy, given to the top college basketball player, the John R. Wooden Award, given to the most outstanding college basketball player, and the Naismith College Player of the Year Award, given to college basketball’s best male and female player. That year he also won the Oscar Robertson Trophy, given to outstanding men’s college basketball players, the ACC Men’s Basketball Player of the Year Award, and the Rookie of the Year Award in the NBA.

Jordan was named one of the 50 Greatest Players in NBA History in 1996 and was named the Sportsman of the Year in 1991 by Sports Illustrated and ESPN named him number one in their list of the Top 100 athletes of the 20th century list. He is credited with inspiring an entire generation to play basketball and many players entering the NBA have been dubbed “the next Michael Jordan.” Many current players name him as an inspiration including LeBron James and Dwayne Wade and in September of 2009 he was inducted into the Basketball Hall of Fame.

Cassius Marcellus Clay Jr., better known as Muhammad Ali, finished his boxing career with a record of 56-5-0 and 37 KO’s. He was the world heavyweight champion three times, and is one of only four men to do so in the history of boxing. Ali only lost his heavyweight title once in the ring with his other belts being stripped after accepting an immediate rematch with Sonny Liston and for his refusal to enter the U.S. Army during the Vietnam War. Ali took part in three of the most memorable fights in the history of boxing, “The Fight of the Century” with Joe Frazier, “The Rumble in the Jungle” against George Forman and “The Thrilla in Manila” in his third fight with Joe Frazier.

Ali defeated every top heavyweight in his era, an era considered the golden age of boxing. In 2007 he was named the second best heavyweight every by ESPN, behind only Joe Louis. Ali was involved in more Fight of the Year bouts and named Fighter of the Year by Ring Magazine than any other boxer. He is also one of three boxers to be named to “Sportsman of the Year” by Sports Illustrated. Ali changed boxing by proving that all boxers do not have to just be sluggers. He famously used the rope-a-dope along with his power, as evident in his famous quote, “float like a butterfly, sting like a bee.” He would also use trash-talk to get in the heads of his opponents, and this along with his unorthodox style made him a cultural icon.

Roger Federer has won 15 Grand Slam titles, most by any man, and is one of only six men to complete the career Grand Slam. He has reached 21 Grand Slam finals, has reached the semi-finals of 22 consecutive Grand Slam tournaments and holds the record of 10 consecutive Grand Slam finals. Federer holds or is tied for the record in 22 categories in Grand Slam tennis history and holds six records outside of Grand Slam play. He received the Laureus World Sportsman of the Year award four consecutive years, (2005-2008) for his tennis ability.

Federer is a rare example of a tennis player who can play on any court and hit any shot. That thought is epitomized in a quote from Jimmy Connors, "In an era of specialists — you're either a clay court specialist, a grass court specialist or a hard court specialist... or you're Roger Federer." His ability to improvise any shot at any time makes him dangerous from any position on the court and John McEnroe has labeled Federer’s forehand as “the greatest shot in our sport.”

All six of the athletes can claim that they are some of the greatest in sports, but what makes them the greatest of all-time? Is it the ability to return from a life threatening disease, or the ability to dominate your sport to the point they change the way the game is played to try and slow you down? With sports fans trying to debate who is the greatest of all-time is like talking politics or religion, if you have an opinion you better be able to back it up.