It's Steffi. 22 slams including winning all 4 at least 4 times each, winning the CYGS + Olympic Gold in 1988, ranking No 1 for 7 years and almost 400 wks, winning 100 tournaments, slams 12 years apart, etc.
There are 3 main arguments forwarded against her but I don't put credence in any of them:
1). Navratilova was greater because she excelled in doubles as well.
- It's weird how this is pretty much the only time when doubles are invoked to raise someone's status. When we are talking about the men's GOAT list, no-one ever discusses the doubles achievements of Fed, Laver, Sampras, Borg etc. No-one ever promotes Court as female GOAT because she has won GS titles in total (singles,doubles,mixed doubles) than anyone else. No-one ever puts the great doubles players like the Woodies, the Bryans near the top of GOAT lists.
Yet Navratilova fans always like to place her ahead of Graf because she was also good at doubles. Frankly it's irrelevant in judging someone's status in an individual sport.
2). The stabbing of Seles inflates Graf's achievements.
- We have to go on what actually happened, rather than what people think 'would have' happened in hypothetical scenarios. It's possible that Maureen Connolly would have won every slam for 10 years if she hadn't been injured in a horse riding accident - but we can't promote her to GOAT because of this. Furthermore Graf was undergoing personal problems in the early 90s (father's tax issues etc.) and still did not have a bad h2h record against Seles. There's no way we can say for sure that Seles would have continued to dominate. Maybe Steffi loses a few slams but that would be compensated by the fact that those she won, she would have won them against Seles, a stronger competitor than what she actually faced.
3). Serena would beat Steffi (and every other great) at peak
- GOAT should be judged on accomplishments, not peak play. Serena is getting reasonably close, but still trails Steffi and Martina at this stage.
Sloane Stephens and Laura Robson: future WTA world No 1 and 2.