Which book is the best to write for women?

Probably romantic fiction is the traditional genre. However this may be, I have heard many reports of women reading books outside their comfort zone and enjoying the experience. This could or should apply to both male and female readers. However the question is concerned with what genre is most popular for female readers. To be safe and capture the largest number of readers, the advice must be to play safe and choose romantic fiction. But my advice would be to write what you are most skilled in, or most comfortable with. Whether for a male or female audience, attempt to bridge both and write the book accordingly.