I think you can measure the level of greatness by considering the number of professional players who play the game and how above the norm the athlete was. So Pele being great a soccer is incredible as most of the planet plays soccer and most gifted athletes will try to pursue a career in soccer over most other sports. Golf and Tennis are more selective sports (in terms of cost), if truly most young men played either sport most likely someone else would be the world champion. Federer didn't really have many great players going against him during his career. much like Serena. Whereas Ali was constantly up against some of histories greatest boxers.
Add to that then not just the number of championships but how the player changed the game and you can measure greatness a little most 'scientifically'. Greatness is more than just championship dominance, it's about how the athlete fulfilled their role with all the pressure and expectation upon them.
Jordan, Ali, Pele, Federer, Serena, Gretzky, Tiger, Phelps, Bolt & babe are probably the top ten. No cricketers, but otherwise it's a representative from each sport and I don't think any woman has dominated her sport quite like Serena.