It is close, but I think Steffi Graf is the best female singles tennis player ever.
Steffi Graf's career is well known to any tennis fan. She won 22 Grand Slam (GS) singles titles and an Olympics gold, and was No.1 for the longest time. She successfully challenged legends of the past (Martina Navratilova, Chris Evert), dominated her generation (Gabriela Sabatini, Monica Seles, Arantxa Sánchez Vicario), and played well against the younger generation (Martina Hingis , Lindsay Davenport, the Williams sisters). She was good on all courts.
The argument basically comes down to two issues.
First is that Margaret Court won 24 GS singles. But Court spent only half of her career in the open era. So it is not a stretch to say Graf did at least as well as Court.
The second, the Seles factor. Some people feel strongly that without Seles's stabbing in 1993, Graf would not have won that many GS. However, no one can predict what might have happened, and this notion might well be a wishful thinking.
Conventional wisdom is that Seles dominated against Graf. This is simply not true. Graf had a winning record against Seles before and after the stabbing. People got the idea from 1990-1992. It should be noted that Seles is younger than Graf, and Graf was also going through a hard time due to injuries, diseases, and legal/family problems then, so the result was not a surprise, but one should never take it for granted that it would continue forever. Seles was new and rising, so people thought she would be the next big one. Did she have it to be it, though? Not likely. Remember, many "next big ones" failed to materialize.
For one, people with the power game like Seles usually do not age well and will have more injuries later in their career. Seles also never moved that well, which would be exploited as opponents adjusted to her play. Seles missed 10 GS during her timeout, Graf won only 6 of those, not a huge advantage as one would think. The most telling part is Seles herself. Her timeout was much more than needed for the wound to heal, suggesting mostly a mental hesitation. But to be great, one needs to be tough mentally and overcomes adversities. Everybody including Graf went through that. Seles could have come back strong in a few months, but she clearly didn't do well to overcome the incident, no matter how terrible it was, especially since steps had long been taken to prevent it from happening again. Seles might look tough on the courts and was still a great champion, but she was not as tough as other legends of the game.
Seles was certainly not napping during her timeout. She was still training, and one would think that she would be fresh and ready to revive her career, minus more than two years' grinding on the tour. When she came back in 1995, she was indeed still a top player, easily got to the final of US Open. There, Graf beat her. She won the next Australian Open (Graf was missing). When they met in the 1996 US Open, Graf beat her again. Thus, while Seles could still beat most people, there was just no real evidence that she could have dominated Graf had she not been out. Especially when Seles was fresh, and Graf was another two years older on the WTA tour age scale. The rust factor can be largely discounted because Seles had little problem with anybody else in 1995 and 1996.
Seles faded after 1996. Graf is older, played much longer, and was still a top player in 2000. So, what might have happened? Assuming Seles maintained her intensity of play, not a given, she could take away a couple of Graf's GS titles during 1993-1995, but Graf might also simply trade her losses to Arantxa Sánchez Vicario to losses to Seles and got a similar number of GS. If Graf suffered a few more defeats by Seles, perhaps Graf would be fresher for more tennis, or she would be more motivated. Seles could have faded in 1994 instead of 1997. We will never know, but based on their characters, Graf was the stronger person. Their head-to-head record says the same thing.
Graf didn't win every GS she entered. With challengers like Seles, Graf could still have won 22 GS, perhaps just in slightly different years and different events.