Most of the sites ow a days use adsense to generate income, which is the highest earning site with adsense...
Good niche also needs to supplement with great CTR. That will give an ultimately very profitable site =)
The Legal niches and insurance niches are also hot for big money clicks but they are also very competitive.
There are specific niche that can pay you high in Adsense such as - credit card / loan / mortgage / medical related / etc. You may consider reading this for further reference Top Paying Adsense Keywords Lists of Top Paying Niches (Adwords/Adsense) | Myseoblog.net
I suppose the simple answer is the site that has the most traffic because if you have a great subject but no visitors there wont be a CTR to speak of.
It is not the site that has more traffic but the site that has targeted keywords either on search engines as in their site. Also, the web copy is important as the ads appearing on the website.
The answer to this question can go around in circles for days. The more traffic you have the more clicks you can potentially get. On the flip side smaller niche sites but lots of them can earn you more than one big site. Whatever works.
Hosting sites, debt consolidation, but the real formula lies with CPC x volume as if you have a high CPC niche but not many visitors it won't make you much money. Volume x high CPC = profits
Sites with most traffic earn the most in my case. The question is not which sites earn the most per click.
Are you serious. I heard that Adsense doesn't accept and publish such ads. Anyway, I think sites with huge traffic earns most from Adsense. It can either be a mobile portal or a car selling site.