My statement is purely because of the rivalry - they won 18 slam titles each at the same time period. Nobody even gets close to that amount during Serena's years, except maybe Henin and Venus.
It should be noted that they would think she was the greatest, because tennis has advanced immensely, especially on the womans side, since then. Serena plays at a higher level than them, purely because that's where the sport is. In 10-15 years there will be plenty female players who play at a higher level than Serena. So Evert and Navratilova claiming such doesn't surprise me.
Serena has 23 wins and 6 finals losses (2 vs Venus, 1 vs Sharapova, then 3 against one or two time winners). Martina has 18 wins and 14 finals losses, 8 of which were to Evert and Graf, two of the greatest players of all time, and 1 to Seles, who won 8 slams in 3 years before the unfortunate event. Evert won 18 slams and lost 10 finals to Martina alone. She lost another 6 finals, including losses to Graf, Margeret Court and Billie Jean King.
Serena has not had to deal with a rivalry anywhere near as comparatively good. And this is shown by the fact that the woman's game is very shallow and unpredictable for the past 10 years or so - random players pop up to have the odd good tournament or two, but nobody has consistently been a threat to Serena, despite the huge amounts of slams that Serena hasn't won in the past decade (as mentioned before, 17 haven't been won by Serena in the past decade).
As for the records, Navratilova won 4 US Opens, and apparently that isn't close to Steffi's 5. Unfortunately the Australian Open was grass until 1988, when she was entering the twilight of her singles career. She still made 2 finals at the US Open after, where she was defeated by a prime Graf. She wasn't great on clay, but she still won it twice and made the final 3 more times to be defeated by Evert.
To be fair, this discussion is moot until Serena retires. She has plenty time to pull well clear of the others...