Well, let's face it, we are all in it for the money,; so who can help with a good answer. Just the niches listed in the order of profitability from the highest paying down.
We, we're in it for the money, however the highest paying niches also have the highest competition. Credit cards, prescription drugs, real estate, medical stuff, and legal niches are the highest paying niches. Just so you know I make more than 99% of the webmasters around here who attempt to try this niches, while I have a low paying niche.
combine those: - traffic from western countries - traffic from a non technical audience - good ad placement
Yes it means not web related, not earning money online, not seo, not webmaster anything, not even tutorials on photoshop or other software. Tutorials are ok if they deal with things such as "how to email photos" or "how to print a photo" etc. Very very basic stuffs, but it's even better if it is not web technical at all. Why? Simply because those non technical people are the best at clicking ads, if they are well located and blended of course, you'll get the highest ctr. On the other hand, a make money online blog will get very little ctr, same for seo related blogs.. Of course it's still possible to earn money from a technical audience, I'm just giving advise for the easiest way to earn money, to target mass traffic from western countries with a good ad placement, that's an easy recipe for adsense earnings.
Pron will get you banned very quickly. Finance, credit, mortgages, education, medical, and green energy will get high paying ads. Though since most of our traffic is USA they pay well.