That is not problem about the hight paying keyword. I mean which site content that you thinks best for site? May be some one tell me that mesothelioma or absolute is good, some one say forex trading or financial and many more. But i still think with most traffic like 10000 page impression and only 500 click at least bring to you more than 20$ per day, so 1 month you can make more than 500$. Its not bad. Do you thinks so?
Category you mean. It seems to be legal/insurance/compensation culture type sites. You know the ones - sites which contribute absolutely nothing to the diversity of the internet.
Best in what way? If your site is about migrating swallows, then content about the mating habits of the Madagascar Lemur would not be the best. The best content for the overall use by visitors is content which is unique and original about a topic of interest. You'll be able to contribute more time and personal knowledge to it if it is a topic that YOU are interested in, otherwise, it's just an exercise in copying someone else's articles on lawyers or some other beat-to-death topic. If you're just looking to wh0re out your site, then content like CountryBoy mentions is "best" But also read his second sentence.
finance/credit/debt type sites pay well but unless you have some serious knowledge and can generate a good following it's tough I hear travel does well also and some medical