You want profitable keywords: high demand, low supply

Keep in mind that some topics attract much higher payouts per click than others. For example, if your site is about topics such as debt consolidation, web hosting or asbestos-related cancer, you'll earn much more per click than if it's about free things. On the other hand, if you concentrate only on top-paying keywords, you'll face an awful lot of tough competition. What you want are keywords that are high in demand and low in supply. So do some careful keyword research before you build your pages.