Kind of hard to measure...
Better than Joe Louis at boxing in his prime?
Better than Irving Crane at straight pool in his prime?
Better than Einstein at understanding Relativity?
Better than George Bush at sucking? (sorry)
I'm not sure how to develop a standard of comparison.
You're absolutely right, it's not exact...it's just a fun thing to think about.
There's a class of "Best in the World," then there's "Undisputed Best in the World by any reasonable person, for a long period of time"...and THEN there's "Dominant Freak who crushes everyone else in the world."
An Olympic Gold Medalist is obviously the first category, "Best in the World."
Wrestling: Kurt Angle
Chess: Tigran Petrosian
Basketball: George Mikan in his era.
Boxing: Lennox Lewis
Military: Hm, let's say George Patton.
Someone who wins more than one Gold medal or has the label of best for a long period of time is the next level-up. These people are usually national heroes and historical/mythical on top of it...and get the label of "Greatest".
Wrestling: Dan Gable (Gold Medalist, afaik went undefeated AND unscored upon when doing so)
Chess: Garry Kasparov (World Champion for many years)
Basketball: Kobe Bryant, Hakeem Olajuwon
Boxing: Muhammad Ali
Military: Napoleon
Even a level above that is the Dominant Freak. A person who not only is the best in the world, but in their prime crushes everyone else on the planet in a scary fashion.
Wrestling: Alexander Karelin (undefeated AND unscored on for something like 10 years. Won everything at his class in wrestling.)
Chess: Bobby Fischer
Boxing: Mike Tyson before going to prison, Joe Louis probably
Military: Alexander The Great
Basketball: Michael Jordan OR Wilt Chamberlain.
Others: Tiger Woods, probably Lance Armstrong, Secretariat, Mozart, Da Vinci etc.
Those are just the people I'm aware of...
The non-competitive ones are too hard to judge objectively. It really comes down to whether Fischer in his prime was more of a dominant freak than the other people in that category. You have to compare his 21-0 streak to Woods' 18-stroke victory, Mike Tyson's run of knocking guys out in the first round, Jordan's 3-peats, Secretariat's 31-length victory...and, well, Karelin's entire career.