This is actually the topic of spirited debate on another forum... I'm willing to buy it given what he has been able to accomplish (not just victories, but each in world record time), swimming and dominating in each and every stroke (Spitz only swam two, not four), he's been doing it for three successive Olympiads now... and clearly elevating his sport beyond where it's ever been before. Then of course you have sheer medal (and Gold Medal at that) count.
Is Phelps the Best, Or Does That Take More Than Gold? 
August 15, 2008By JERÉ LONGMAN
BEIJING — Michael Phelps has won 11 career gold medals, more than any Olympic athlete in any sport. But does that make him the greatest Olympian ever or, remarkably but somewhat less loftily, the greatest swimmer ever?
That is a subject of debate here at the Beijing Games. Are Phelps’s accomplishments in the pool more impressive than the nine gold medals won by Carl Lewis and Paavo Nurmi in track and field? Or greater than Eric Heiden’s five victories, from 500 meters to 10,000 meters, in speedskating at the 1980 Winter Olympics in Lake Placid, N.Y.?
Does Jesse Owens’s refutation of Aryan supremacy, by winning four gold medals before the Hitler viewing stand at the 1936 Summer Games in Berlin, give him a moral validity that supersedes Phelps’s athletic legitimacy?
For his part, Phelps is avoiding the debate.
“I just swim,” Phelps said Thursday. “I don’t think about it.”
David Wallechinsky, the vice president of the International Society of Olympic Historians, gives the nod for greatest Olympians to Lewis and Nurmi. His point is this: It is much easier to win multiple medals in sports like swimming and gymnastics than in track and field, because there are more individual events. And fewer countries produce elite swimmers than runners, making track a more democratic sport.
Lewis won the long jump in four consecutive Olympics, along with five golds in the 100- and 200-meter sprints and the 4x100-meter relay. Runners compete vertically. They pound their bodies harder, and must work their hearts and muscles harder, than swimmers, who compete horizontally.
Swimmers recover quicker and, during major international competitions, generally compete in fewer rounds than runners. Lewis not only sprinted but subjected his body to the enormous gravitational forces of the long jump, where landing was the equivalent of jumping out of a truck at 25 miles an hour.
“Very few people have won the same event four times; only Carl Lewis won the same event four times and won nine total gold medals,” Wallechinsky said.
Nurmi, the great Finnish distance runner, won nine gold medals and three silvers in the 1920s. He won five gold medals at various distances (some discontinued) at the 1924 Paris Games, including the 1,500 meters and the 5,000 meters, which were held about 90 minutes apart.
“That’s almost inconceivable,” Wallechinsky said.
Phil Hersh of The Chicago Tribune, who has covered 14 Olympics, rates Phelps sixth on the list of greatest Olympians. His top five are Lewis; Nurmi; the gymnast Larysa Latynina, who won 18 Olympic medals, nine gold, for the former Soviet Union in the 1950s and 60s; the canoeist Birgit Fischer-Schmidt, who won 12 Olympic medals, eight gold, for the former East Germany and unified Germany from 1980-2004; and the rower Steve Redgrave, who won a gold medal in five consecutive Olympics for Britain from 1984-2000.
“Lewis didn’t have as many relays as Phelps; what if he would have had a 4x200-meter relay, or some kind of medley relay?” Hersh said. “Nurmi, because of what he did and the fact that he lost an Olympics because he was declared a pro. Latynina, just look at her record; it’s unbelievable. Fischer-Schmidt competed over a 24-year period in canoeing, which is not a child’s sport.”
Di[COLOR="Black"]c[/COLOR]k Pound, a member of the International Olympic Committee from Montreal and an Olympic swimmer at the 1960 Rome Games, said it was difficult to compare Olympic champions — “it’s like comparing cave drawings” — but that Owens stood out to him.
“He stood for a lot of things,” Pound said. “He’s one of the greatest. I’d like to have 10 athletes to play with instead of one. But in terms of performance, if Phelps pulls off eight golds, he’s been pretty dominant in a pretty hotly contested sport.”
John Powers of The Boston Globe, who has covered 16 Olympics, said he gave the nod to Phelps as the greatest Olympian. Phelps swims all four strokes, compared to the butterfly and freestyle that Mark Spitz swam when he won seven gold medals at the 1972 Munich Games, Powers said. Plus, Phelps has had to swim more preliminary rounds.
“In terms of the pressure he’s under here, winning eight overall medals last time in Athens, seeking eight golds this time, I’d go for Phelps,” Powers said. “He’ll have 17 races here. And the sport has become more global. Spitz’s biggest competition in 1972 was his own guys. Phelps is going constantly, morning and night. He’s had to be good twice a day. Track guys don’t do it every day. And Phelps is doing it at a time no one’s done before, with finals in the morning. I’d put him up there.”
Perhaps the best argument in Phelps’s favor is that he is swimming a range of distances that convert to track races from a quarter mile to a mile (sprinting is about four times faster than swimming). No quarter miler is also dominant as a miler.
Phelps has superior aerobic capacity, compared with track sprinters, gained by swimming more than nine miles a day during peak training periods, said Genadijus Sokolovas, the director of sports science for USA Swimming.
“Track and field sprinters come to their races from short-distance training,” Sokolovas said. “They do a lot of speed work, not as much endurance. As a result, sprinters don’t recover so well. They don’t have enough aerobic capacity. If they trained differently, it might be a different situation. Michael’s ability to recover after such high volume training and race is amazing.”
Only 23, Phelps has a chance to become the greatest Olympian, Wallechinsky said.
“If he returns in London in four years and wins a few more gold medals, that would show a greater longevity,” he said.
http://www.nytimes.com/2008/08/15/sports/olympics/15longman.html