No, article directories (if that's what your referring to) are not good. I suggest you do a SEARCH and read about the latest Panda update. As far as "the best", you don't wanna start with "the best" you want to start with something that has a medium level competition. Use Google Adwords keyword tool to find the right niche for you and it's competition level.
Health,Forex,Insurance and Tech topics are best topics to earn from Adsense as these all are high paying niches in Google Adsense
I've heard that health and beauty topics are good for adsense. YouTube users lisa3876 talk about Adsense a lot. huser Gnomelocker has also talked about adsense before.
We can give you the list of all the top earning topics but will you be able to write articles on those topics. I agree with F--K and stick with what you know and write about it. techbytes
Write about what you know better and once you get decent traffic you should be getting attractive revenue. There's no such specific field which will pay you big cheques
"conversion" is the word you need to keep in mind. It is best seen on adwords. Learn about adwords first. Otherwise you can never know the reality about adsense.
There are lots of good niches but hard to say which one is the best. Just pick one for example "acne treatment" and you will get the feeling.
Technology, AdSense , Forex Trading etc Try adwords tool, to get the information about releated keywords and find a suitable topic, do some homework.
I've heard that adsense earnings from ads about law/lawyers and weddings, bride's dresses, etc make really good money on clicks