There are three kinds of articles: the Review type, the How To type and the Information type. Of these three review type articles will always earn you most.
People read these articles when they are looking to buy, with their credit cards out and ready, almost.
The Perfect Article will have high value clicks, no competition, be enjoyable to write and have millions of searches per day. You have to compromise on at least two of these factors.
Go to your Google Adsense account – Opportunities – Key Words Tool – Advanced Search Options – All countries. Type in your search term. Make sure All Related Searches box is checked. Search
Google does its stuff and gives you up to 800 related search terms. Order these by clicking on Global Monthly Searches. Go to the last page. You are looking for around 1,000 searches, but with the competition bar about half way across.
If necessary go forward a few pages. Click on any you find, this will open the Google search page in a new tab. Check out the results, if you find any which are not highly ranked then you have a good chance with an article that includes that search termin the title.