We doing international or just US. We doing all sports or just the more main stream ones. I mean Ledecky is the most decorated US female athlete if you go by Olympic medals and her 9 golds is tied for the most all time, it is her and Soviet gymnast Larisa Latynina (Latinina has 18 total to Ledecky's 14 - Marit Bjorgen has 15 for 2nd most all time). Ledecky won the 800 in 4 straight Olympics winning it first at just age 15. So been around awhile and won a lot. If just Americans' how do you exclude her?
As for tennis, I'm not sure Serena is better than Steffi overall. I'd probably lean Serena because of doubles, but as singles players, Graf was ranked 1 longer, won more overall titles, and had just 1 less GS win. Graf also medaled in singles at 2 Olympics including the Golden Slam, while Serena only had the 1 singles medal (3 doubles). So given Serena played a bit longer and was a much more prolific doubles player, I'd lean towards her, but wouldn't find any issue if someone thought Graf was better (just played less - i.e. Graf is Jordan, Serena is Lebron).