I don't believe "the greatest" is one person. Just like in the men's game, it's a group of people who can all stake a claim. My GOATesses are as follows:
Chronologically...
Suzanne Lenglen; unprecedented dominance, with a 287-1 win-loss record over nine years from the end of WWI until she turned pro. Won nine tournaments in her career without the loss of a single game.
Helen Wills Moody; rivals Lenglen for dominance; 180-match win streak over seven years without losing a set. Won 19 of the 24 slams she played in.
Margaret Court; queen of the numbers. 24 singles slams, 64 in total, a CYGS in singles and two in mixed, multiple major slam records (six singles in a row, nine in a single year, etc etc). Also has the highest win-loss percentage in the Open Era, for those who think she was just an amateur champ.
Billie Jean King; overshadowed numerically by rival Court, but still has 39 slams to her name, equal third all-time. Also made immense contributions off-court to ensure women's tennis thrived.
Chris Evert; a machine of consistency. 18 singles slams despite skipping multiple majors in the '70s. Would almost certainly have 22+ otherwise. Ridiculous record of reaching semis.
Martina Navratilova; "the greatest singles, doubles, and mixed player who ever lived" (BJK). Insane domination across all disciplines, and owns multiple Open Era records e.g. longest win streak, best seasons, etc etc.
Steffi Graf; unique Golden Slam, and likely the best singles resumé of all since she took much less time to accumulate her titles than her rivals.
Serena Williams; Has the Open Era record of 23 singles slams, and is equal third on the all-time list with 39 overall. Only two mixed titles short of a boxed set. Also has amazing longevity; 21 years between first and last slam finals so far.
Honorable mentions to three "what ifs"...
Alice Marble; in her last two years she was 111-0 in singles and won four successive Triple Crowns, but WW2 and turning pro ended any chance she might have had to emulate Lenglen and Wills Moody's numbers. Also has the most batshit insane autobiography of any champion.
Mo Connolly; forced to retire aged only nineteen after breaking her leg while horseriding, by which time she'd won 9 slams in succession, including the first CYGS by a woman in 1953.
Monica Seles; who had 8 slams at nineteen and could have exceeded Connolly's teen records but for one deranged Graf fan.