Okay, in reality, it is impossible to unequivocally answer this question.
This is sports, however, where (as I have written before) the difference between reality and fantasy becomes blurred.
My fantasy would be the invention of a time machine—the only way we could seriously attempt to fairly and accurately answer this question.
Even then, it would not be as simple as it might appear to be at first glance.
In team sports, for instance, if your goal is to determine the best individual player in a sport, this still might not be so clear, because there would still be other individuals competing on the court or field.
Even if you were trying to determine something as specific as of the two, which basketball center was better, Bill Russell or Kareem Abdul-Jabbar (who never played against each other), for example, you still may not have a definitive answer.
After all, Russell and Wilt Chamberlain played against each other over 100 times, and is still one of the most debated questions in sports history. (By the way, the answer to the question, "Who is better, Russell or Chamberlain?", it's Jabbar, of course.)
If you were trying to determine who was the greatest in an individual sport, it would still not be definitive, even if you limited it to only one criterion or question, such as: Who would win head-to-head, if two athletes played each other in their prime?
It seems on its face to be a fair test for determining the best of all time in an individual sport. But is it really?
For example, you know as well as I do, that if we beamed Bill Tilden in his prime (circa 1921) onto a court to play Roger Federer in tennis in 2007, Federer would win.
Does that mean that Federer should automatically be ranked higher than Tilden when we are trying to determine who the greatest tennis player of all time is?
I think almost of all of us would answer something like, "it is not that simple." If it were, we would not have to ask who is the greatest of all time in sports like swimming or track and field, because whoever is the current world record holder would be the greatest of all time (for the most part) by default.
We all know that today's best athletes are superior to yesteryear's best athletes, because of better diet and training (especially weights). Also, in most cases, today's athletes have spent more hours practicing and competing throughout their careers than their counterparts in the past.
These are huge advantages, especially when the athlete starts playing the sport at age two—ever hear of Tiger Woods?
In fact, it is fair to say that as of 2007, each generation of athletes is clearly superior to the generation before.
An exception to this might be a sport which is currently a lot less popular than it was previously (examples in the U.S. would include boxing, bowling, and to a lesser extent, baseball), since significantly fewer great athletes have chosen to compete in that sport.
In the future, the difference between each successive generation might be marginal or negligible, but for right now, the difference is measurable. So how do we account for this fact when determining who is the greatest of all time in a particular sport?
There is, of course, no right or wrong answer to this, but one thing is clear: How much "allowance" you give to the earlier generations' athletes for the advantage that the modern athletes have will significantly affect any ranking that you have for greatness in a particular sport.
Since today's athletes have the above-mentioned advantages, how about we limit our evaluation to only one criterion: How much better was an athlete than his or her contemporaries?
Again, this seems to be fair on its face, but what if the athlete's contemporaries were especially weak or strong? How do we know for sure that they were weak or strong?
After all, they only played against their own generation. Even if we could tell for sure, how much should we account for this factor?
Let us try another specific criterion: Which athlete was considered the best in his or her sport for the longest period of time? Unfortunately, you run into the same problem—namely, that the strength of their contemporaries could have affected their length at the top of their sport.
It looks as if, time machine or no time machine, the only way we could even attempt to answer the question is to use a combination of criteria or factors.
Whatever criteria that we use, it is safe to assume that: 1) the most recent athletes will probably place higher than they should; and 2) your age will affect your rankings.
Plus, how accurate will an evaluation by a 20-year old be, given that he only witnessed as few as 10 or 20 percent of some sports' athletes?
How do we account for the fact that almost all of us have not seen all the best athletes in a particular sport? And how can we fairly compare an athlete that we never saw with one that we saw compete on a daily or weekly basis?
It seems rather obvious that there are no good answers to these questions. However, that does not mean we cannot at least try and answer our ultimate question.
In my article, "Who are the 25 Greatest Athletes of All Time?", I suggested 10 criteria for trying to answer this question. Some of the criteria do not pertain to our question at hand so I eliminated them, and we are left with the following suggested criteria:
1. What were the athlete's accomplishments in their sport? Things to consider are titles (especially majors, olympic, or world titles), championships, records set, rankings (in individual sports), career statistics, all-star selections, awards (especially Player of Year awards and MVP awards), and the length of their careers. Also, did the sport's rules or equipment changes affect the athlete's statistics?
2. For how many years were they considered the best in their sport? How much better were they than contemporaries? How weak or strong in ability were their contemporaries?
3. When you watch the athlete perform, do they do things that other athletes in their sport cannot do? Or, to put it another way: How exciting is the athlete to watch because of their amazing athletic ability?
4. How much impact did the athlete have on their sport or the sports world in general?
5. How much did injuries or a lack of opportunity limit their accomplishments?
6. How consistently great was the athlete?
7. How much did the athlete's mere presence intimidate their opponents because of the athlete's dominance? In a team sport, did the athlete's mere presence or greatness make their teammates better? Also, in team sports, did the athlete make their teammates better in other ways (e.g., leadership, teamwork)?
I think most of us use most or all of these criteria (and any that I may have missed) either instinctually, or perhaps after some thought.
You will soon realize that how much weight you put on each of these criteria and factors above will significantly affect your answers (you can give your answers at my site, UltimateSportsRankings.com).
Ultimately, we may never come up with a definitive answer, but at least, we can have some fun trying!