I've discussed this with regards to the men's GOAT, but I think we have to consider two things:
One, the game over time tends to evolve. There's no real way to test this, but we can assume that tennis today (or baseball or football, etc) is better than it was 30 years ago, which was better than it was 30 years before that, and so on. By "better" I mean played at a higher performance and skill level.
Two, we can and should assume that if Margaret Court (for example) played today, she'd play a more modern style - and it is likely that her natural skill and disciplined training would lead her to become a great player now. As good as she was in the 60s? We can't know that. But the point is, Bjorn Borg wouldn't have used a wooden racket today. Babe Ruth would likely have employed modern training techniques (and perhaps stayed away from the bottle) if he played today.
Which leads me to my conclusion that the only way to really give the term "GOAT" justice is to look at player's relative to their eras and then compare how great they were relative to their eras. In other words, when we ask: "Who was greater, Pete Sampras or Roger Federer?" We have one match to go on, but its not enough. What we can do, however, is look at how dominant Sampras was during his career, especially during his peak and against his peers, versus the same for Federer.
Turning to the WTA, the only reasonable approach to this, in my opinion, is to look at their contextual greatness. How great has Serena been over her career relative to the field? How great was Graf over her career? Etc. It is useless to ask, "who would win a match, prime Serena or prime Steffi?" We just cannot know.
Which leads me to my own choice of GOATess. As with the men, its hard to compare the further back you go because the game changed so much. But I think we can safely say that Margaret Court was the GOATess up until the 1970s, and then eventually Evert and Navratilova surpassed her. Actually, in women's tennis there's clear "line of queens" over the last 50 years or so, with pairs of women sometimes flip-flopping the crown back and forth:
~1960-73: Margaret Smith/Court and Billie Jean King
1960-66: Smith (Court)
1967-68: King
1969-70: Court
1971-72: King
1973: Court
1974-87: Chris Evert & Martina Navratilova
1974-80: Evert
1981-82: Evert/Navratilova
1983-87: Navratilova
1988-96: Steffi Graf & Monica Seles
1988-90: Graf
1991-92: Seles
1993-96: Graf
1997-2001: Various
Here you have a mixture of Martina Hingis, Lindsay Davenport, Jennifer Capriati, and Venus Williams holding the title for short periods of time. Hingis and Davenport finished #1, but neither were dominant - except for Hingis for about a year and a half
2002 - present: Reign of Serena...sort of
2002: Serena
2003: Serena/Henin
2004-05: Mix (Davenport was #1, but not clearly dominant)
2006-07: Henin
2008: Mix
2009-10: Serena
2011-12: Mix
2013: Serena
I wrote up all of the above to make a point. Serena's reign of greatness has been both the longest of any great player - having won Grand Slams over a remarkable 15 year span (and counting) - and the most erratic - in 5 of those 15 years she didn't win a Slam at all, mainly due to injury and personal problems.
If Serena had been more consistent then I think this discussion would be a moot point. She'd probably have 20-25 Slams by now and have surpassed the others as the clear GOATess. But this isn't a game of what if, and consistency is a major component of greatness.
I think Serena, at her very best, is the greatest woman tennis player in the history of the game. That said, not only could she not maintain her best for long periods of time, she had a hard time for an entire year, even being somewhat erratic in her best years. In other words, despite her peak greatness her best years weren't as good as four or five of Steffi's best, and she certainly never had a span of years like Martina's 1982-87, and was not nearly as consistent as Chris Evert.
All things tolled, right now my list would be:
1. Graf
2. Navratilova
3. Evert
4. Serena
5. Court
Serena has a chance of passing Evert and Navratilova, but it would require at least a few more Slams. While she's only one behind them in Slam count, we have to take everything into account, including competition. While we could argue that the level of tennis today is higher, Serena never really had another truly great player playing at her peak during the same period of time. Henin was close and Clijsters had her moments, but Graf first had Navratilova then Seles, and Martina and Evert had each other, and Evert had King then Navratilova.
She still could become the GOATess, but she'd really have to pass Steffi's record, in my opinion, and I have a hard time seeing her winning 5 or 6 more Slams.
:clap Serena can never overtake her when all is said and done and I wish people would stop trying! You give her excuse of injury, but people overlook and discount Seles being stabbed in the back! I have no words for the hypocrisy and naivette! - I jumped on the Sampras bandwagon for a while even though Laver had 2 Grand Slams to his name; 1 in pre-open tennis era! Federer made it all moot when he overtook Pete so completely taking a FO; playing several finals while Sampras never got there! :nono
