The cricket player Donald Bradman [1] is the outlier among outliers. Even compared to greats like Michael Jordan, the statistical significance of his achievements almost defy belief.
If you're willing to branch out beyond physical sports, Marion Tinsley [1] is perhaps even stronger, losing only 7 games in his 45-year career of checkers.
I don't think checkers is comparable, since there's no luck component in checkers. In a physical sport with random winds/ground conditions/ball imperfections/bat imperfections/etc, you have to be that much better than your competitor to beat them consistently (and that margin varies by sport, where baseball, for example, has many more random factors than tennis).
This is a false conception people have about sports like checkers, chess, go, etc. It depends on how you think about luck. Sure, if you're an omniscient being who knows the entire decision tree for a given move in [checkers/chess/go/etc...], there's no luck, in the Monte Carlo sense. But, except for simplified positions, nobody is fully aware of the consequences of their move in these games. This "fog of war" is a good analogue for the random winds/ground conditions/ball imperfections that you mentioned in other sports. It's often why great players in these games sometimes lose to far worse players.
Checkers is a solved game. It proven possible to always not lose a game if you play correctly from the beginning. As such high level games feature many draws and few wins. Marion Tinsley also didn't compete in the World Championships for 20 years in the middle of his 45 year streak, during which time Walter Hellman was world champion for 17-odd years and only lost 7 games at the World Championships during that time.
Marion Tinsley may be the best checkers player of all time, but his achievements are nowhere near unbelievable given the nature of the sport and achievements that are similar to his. Statistically, Donald Bradman is much more of an outlier when compared to other people who have played his sport.
You should check out this [1] titled "The most remarkable graph in the history of sport" to get an idea of just how much of an outlier Bradman is. Read the whole thing, it's brief, but a quick excerpt:
To understand how Bradman’s 99.94 average compares with other batsmen,
consider that a typical topflight batsman has an average in the range 45 to
55. Batsmen with averages above 55 are once-in-a-generation phenomena who
dominate the entire game. After Bradman, the second highest average in
history [2] belongs to South African Graeme Pollock, with 60.97, and the
third highest to West Indian George Headley, with 60.83.
Prior to the 1932-33 Ashes Tour(1) his test average was 112.29 after averaging over 200 per innings against South Africa and dropped to a mid career low of 89.55 as a result of the English tactics. Very impressive.
> When she retired in 1981 at the age of 40, McKay had gone nearly 20 years undefeated (with the only two defeats to her name occurring at the beginning of her career).
During the recent radio coverage of the current Ashes tour, between England and Australia, I heard a great quote about Sir Donald Bradman, the famous Australian cricket player. It went as follows: In the 1980s, England's fast bowler Bob Willis had the fortune to meet Sir Donald Bradman, legend of cricket. Bradman had been the dominant player from the 1930s and 1940s and the Australian had amassed stats that are unlikely ever to be bettered. Willis was keen to get The Don's view on what he might have averaged if he had played in the modern game. Bradman looked at Willis and replied that he reckoned that he would have averaged about fifty runs per game. Willis expressed surprise, having thought that the great man might have expected to have averaged more. Bradman looked Willis in the eye and then responded along the lines of "Well, I am in my seventies now!".
I have watched on TV Ian Chappell narrate this same story, as an incident between him and Bradman when they met at a party celebrating Bradman turning 90. Very likely apocryphal.
Bradman survived the 'Bodyline' (http://en.wikipedia.org/wiki/Bodyline) series where the bowlers (pitchers in American parlance) literally targeted the body to injure the players and to hamper Brandman's prowess. It had the desired effect (Brandman only averaged 56 for the series as opposed to his career average of 99.94) and resulted in multiple injuries.
As long as he gets the advantages of modern nutrition, fitness conditioning, and analysis, then I'm pretty sure Bradman would tower over modern day players.
Don't forget, in Bradman's time, there were no covered pitches, leg side theory was legal, batmen didn't wear helmets or much in the way of protection (and bowlers weren't that much slower than they are now).
[1] http://en.wikipedia.org/wiki/Donald_Bradman