What is this current obsession with stevia? It has a very strong, pretty much unpalatable taste that even strong espresso can't mask.
The best AS for me is sucralose (Splenda). I can't tell it apart from sugar. I've also never seen any results about it that would concern me, and it's been studied more than almost anything else we eat.
[1] http://news.psu.edu/story/284556/2013/08/20/research/multipl...
Oddly enough, after so many years, I don't like the taste of sugar any more, it seems 'off'. Aspartame is fine.
In the current zeitgeist, 'natural' is assumed to be better.