US Open, page-30

I guess it depends on what your definition of the modern era is, but for mine Martina Navratilova is my #1 with daylight in second place.

Check out the comparative career stats as follows:

Serena Williams 72 singles titles (including 23 majors) / 23 doubles titles (including 14 majors) / 2 mixed doubles titles (both of which were majors) / 3 team competition titles.

Martina Navratilova 167 singles titles ( including 18 majors) / 177 doubles titles (including 31 majors) / 10 mixed doubles titles (all majors).

Serena Williams has won a total of 100 career titles including 39 majors.

Martina Navratilova won a total of 354 career titles including 59 majors.

https://en.wikipedia.org/wiki/Serena_Williams_career_statistics

https://en.wikipedia.org/wiki/Martina_Navratilova_career_statistics