In any site, if you write voluminously (quantity) with a semblance on quality, you have a fair chance of earning in the long run. Quantity is the key.
This way, you will find any writer writing at his maximum in one site will always boast that the site is best.
As for me, it is Helium.com. I have close to 500 articles there. I get 2.5 dollars per article as upfront payment for every new article I write at eligible channels. Further, I am currently getting $1.5 per day as page view income.
But to get at this, it requires effort, time, quality, patience, commitment to the site .....
Concentrate on one good, established site. At the same time, keep one or two extra sites too to fall back on if the first one fails. This is my advice.