These questions ALWAYS come back, inevitably, to opinions. Well-argued and logical perhaps, but opinions nonetheless. Here's how I select a GOAT---and I freely admit that it is an imperfect method and fraught with "Yeah, but..."s
I evaluate a player at his or her very best---the highest level of play he or she ever reached. Then I factor in how much of his or her career was played at that level. Finally, having narrowed the field to a handful, I ask myself how those individuals would do head-to-head, all playing at their greatest playing level. Some extrapolation here, admittedly.
I saw Martina go through an entire calendar year with a single loss---one. Her record that year was something like 87-1. Total domination. For years in the middle of her career, she dominated everyone but Chris. She and Chris are even with slams, so that isn't in play. Chris was unbelievably consistent with her results for a decade and a half.
Steffi was just as dominant at her peak---and she won 22 slam titles, not to mention the Golden Slam. She stands alone with that accomplishment.
I think Monica would have been in the mix had she not been stabbed, but that affected her career so drastically that she was never the same after that. So modern day, the first three I named.
A generation earlier, Margaret Court won 24 slam titles. 24. Two dozen. It makes no difference where they were played, or who else was in the draw, or anything else. Every other player of her day could have played in every single slam Court did. If they chose not to, you cannot penalize Court for that. She may very well have rolled through a full field every year anyway. She showed up every tournament and she did what she had to do. Period. All-time slam leader has to carry some weight.
Based on everything I have read and heard from those who saw her play, Maureen Connelly (Little Mo) was perhaps the greatest/most talented ever to play the game. She just didn't have the length of career to make the same GOAT argument.
Finally, Suzanne Lenglen may always be underrated because of the time in which she played, but she not only won 2 gold medals and a bronze at the 1920 Antwerp Olympics, but between 1919 and 1926 she lost only ONE match!!! Think about that for a minute---one loss in seven years against all comers. No modern player has ever approached that kind of domination. And she made playing tennis look like ballet on the court---while obliterating all her opponents.
Assuming you can never really compare one generation of players to another and can only look at individual accomplishments against their own cadre of opponents in the time they played, I would put Serena in the conversation, but there are numerous arguments against her as well, many of which have been spelled out by earlier posters.
Best I could do would be narrow it to two and they are too close to separate---Martina and Steffi. Incidentally, I think that at their best, they would both have strong winning records over Serena. They were thinkers on the court; Serena is a physical force out there, but not particularly cerebral. IMO Serena's mentality works against her. In spite of what she says to keep the peace with the tennis public, she clearly doesn't respect her opponents nor their ability. It isn't confidence---she was instilled with a rather brazen cockiness by her father as a survival technique based on when and where she and Venus grew up. Maybe it helped her then. It hurts her now. She honestly doesn't believe she can get beaten. She convinces herself that if she loses, it is because she played sub-par, not because an opponent outplayed her on their own merit. That mentality causes her to question and challenge linespeople, umpires, everything but her own weaknesses and flaws. She doesn't think she has any. She doesn't develop a more diverse game because she doesn't believe she needs to. The physical potential was always there to be the GOAT---but the mentality never was. It still isn't.