Again, a tough call! 
All these women are legends of their time.
Growing up I started following tennis because of Steffi Graf. Her powerful forehand and charm were equally captivating. 
She is probably the only tennis player to hold the “Golden Slam,” winning all four major Grand Slams (Australian Open, French Open, US Open and Wimbledon) and an Olympic gold in one calendar year.
At the same time I totally agree with @Nidhi regarding Billy Jean King. Who else could have challenged the flamboyant Bobby Riggs in the “Battle of the Sexes” and crushed his chauvinistic attitude?