Step 1: Recall what counts as a Grand Slam singles title.
The four Grand Slam events are the Australian Open, French Open, Wimbledon, and US Open, and a singles title is won by beating every opponent in that draw.
Step 2: Compare the career totals of the players listed.
Steffi Graf has 22 titles, Martina Navratilova has 18, and Billi Jean King has 12, all excellent totals but not the highest recorded.
Step 3: Check Margaret Court's total.
Margaret Court won 24 Grand Slam singles titles from the 1960s into the early 1970s, a record that still stands as the most of all time for women's singles.
Final Answer:
The player with the maximum Grand Slam singles titles is Margaret Court. \[ \boxed{\text{Margaret Court}} \]