That's an interesting and difficult question.
I think Hingis' had a crazy amount of natural talent and inate intelligent skill but not the athletic ability.
Steffi Graf had amazing natural athletic ability, I heard she could have turned to any athletic discipline, she could have been like Heiki Dreksler (German Olympic heptathlete and long jumper). I do find Graf hitting slice backhands all day slightly infruriating.
Srena Williams has an unbelievably good technically pefrect serve; her ability to apply slice and topspin at pace is a lost art in both mens and womens game because players don't practice it as much due to medium paced courts now, but she came from a different era. Her movement is great but she can look clumsy, but as a result comes up with great improvisation, like a great batsmen in cricket would who has not learned the classic technique.
Justine Henin would be included for me; the best one hand backhand I've seen on a player, I was lucky to see her play many times on grass, clay and indoors and she was always fantastic to watch. I remember the first time I saw her on tv in 2000 at the Canadian Open and I couldn't believe a small girl was hitting a backhand like that!
Petra Kvitova today has the potential to be one of the best. When I saw her live, she looks like she is just touching the ball and it flies off her raquet for a winner!! Incredible natural power, she needs to improve her movement and will to win and she could be very special. She loves going to net, which in today's times is something to be cherished.
Difficult question but the front runners for me would be Graf, Serena, Henin and Navratilova. Hingis just misses out due to lack of natural athletic ability.
Players I would give special mentions to would be Amelie Mauresmo, Kim Clijsters, Venus Williams, Hana Mandlikova.