The best ever is almost always the most recent dominant #1, as improvements in technology, diet, and so on raise the level of performance.
Results wise, Serena does still have a way to go. I know that she doesn't have to get all the way to Navratilova's 167 titles or to Evert's 157, or even to Graf's 107, but she surely has to be closer than she is now. Unless she were to end up on 80 titles or more, that would at least be something of a case against her, too. It is harder to win titles en masse than it was, because the game's more physically demanding, but Serena only just overtook Davenport's title haul.
Also, Serena has still only been the year-end #1 three times. I know that the rankings are flawed and that she may well have merited the top spot in 2008, 2010, and 2012 as well, but we have the rankings for a reason, and so should grant them some credence. [I don't think you can make a good case for Serena deserving #1 in 2003. You can argue that she would have merited it had she not missed half the year injured, but the fact remains that she did miss half the year injured, and as a result Henin put together a better body of work. Even missing half the year injured in 2010, Serena still probably put together a better body than did anyone else]. So, I'd say that Serena needs to be year-end #1 at least two or three more times.
As for Serena's longevity, it may still be too early to tell. She did just give us the best year of a 30-something since (at least) Court's 1973, and so she should still be #1 next year. But compare with Navratilova: in 1989, at 32, Navratilova was still quite a bit ahead of anyone but Graf. But by 1992, she'd not only slipped well behind Seles as well, but had been caught up by the likes of Sabatini and Sanchez. If we compare with Serena, we could say that part of Serena's dominance is the result of not having a younger rival on a par with Graf or Seles. Azarenka is surely on a par with Sabatini and Sanchez, though, so by 2017, when Serena is 35 until late September and Azarenka is 27 until late July, it's likely that Azarenka will be just as good as Serena is. And there's still a chance that, by 2017, someone better than Azarenka will have emerged, and if that has happened, that woman will surely have an edge on the 35-year-old Serena.