The highest click rate tend to be from niches that have a high value on the products or if the competition is aggresive. For example if you look at the diamond market, the rich have not been so affected by the credit crunch so its still a bouyant market. If the demand is still there and the competition is good then you'll get a better click rate. Or if you look at niches like car insurance, its something everyone who has a car needs so the competition between the insurance companies is aggresive. Click rate on car insurance is very nice!
A high ctr - click through rate can be achieved by blending, contrasting or complementing the ads with the contents depending on the type of your niche and placement of ads at vantage points in the site.
Ads on health niche websites, placed above the first fold, on the right side of the page pay well. Page Rank matters.