Serena is atleast at her best 50% of the time, unlike Safin who is like 0.5% of the time. Serena also atleast has numbers close to the other GOATS. 17 slams vs 18 for Chris and Martina, the real number of 18 for Court, the real number of 20 max for Graf. I think in a series of matches Serena would win over all those, and that takes into account her off days where she would lose some. The reason she has less slams is her career isn't over yet, tougher depth today, some bad luck with injuries and personal tragedy, and most of all that all these women at their best will win and so Serena having a few more off days misses out a couple more times. However if they were all together this when all at their best they win would only hold true still for Serena, not for the others.