That is a very interesting question, and I think very hard to answer because of all the injuries. Davenport is #1, but I think she is benifiting from a lot of players being hurt. But I do believe she is the cleanest striker of the ball, simply amazing how crisp she hits it.
Henin has mad skillz (and mental problems, cheater!) and she fights like a dog. If she was still healthy I think she would easily be number 1. But to say she is the best on tour right now is silly because she is not even playing.
Serena, too streaky for me to say she is the best.
Sharapova, not yet, but getting there. This girl has everything and she is very young. If she keeps getting better this question will be a lot easier to answer!