the best CTR is 100%. maybe you should focus more on content and promoting your site rather these crazy ideas of CTR
testing is the best, one channel per unit, and there you go... you will know exactly what's going on, tweek, increase, tweek again, etc...
If you have 100% CTR then there is something wrong, that is practically impossible. But the higher your CTR, the better, I would say you should aim for a 25 to 30% CTR, that would be awesome.
While most of the publishers with adsense are content with 2-5% ctr, if you manage to get a double digit it would be considered as great.
When doing any kind of marketing or advertising, you've got to get inside people's heads. You asked what niche has the highest CTR rate. Before delving into that, ask yourself what motivates people to click on ads, and therefore create a higher CTR. Many things can make people click on ads. Boredom. Genuine interest. Desperation. Even fear. Out of those four, desperation and fear are the best. Bored people move their attention to different subjects quickly, and most likely won't even think about buying the advertiser's product. Genuinely interested people might buy, but will probably be more interested in information than a product. Desperate and fearful people will grab at any opportunity that they think might help them. Ads are one such opportunity. So, you want to run a site in a niche that attracts desperate and fearful people. Think of what can make people feel those emotions. Financial troubles, legal troubles, health troubles, etc. Those are the niches that will have the highest CTR, and probably a nice cost per click ratio too. They're more likely to buy an advertiser's product, because they will try anything, due to fear and desperation. In my opinion, that's the basic psychology of it all. Not pretty, but then again not much is in life.
Usually it's about 1.5% but there are ctr on certain sites as high as 4% and above. Usually it is niche related and that is also in part influenced by your placement.
LOL...one might think...but that could lead to problems... 100% is NOT the highest CTR. Imagine, if you will, that someone brings up one of your pages and then clicks 4 ads...