some niches are extremely good like info tech and making money online but CTR tends to be really LOW as webmasters and IT people are generally ad blind. This is mentioned thousands of times in other threads. for me, i rather go for non it related niches such as flowers, furniture, mortgages, health, travel, electronics, insurance. for low paying non it related niche which i noticed so far must be entertainment related eg. games, videos. there' still too many people around the world even those in IT line who still don't seem to know much about Adsense and continue to click the ads. even for me, some of the ads i seen on other sites can be very very tempting for me to click upon.
You need to find your own niche it's just simple research and trial and error. There is nothing more to it. All the niches that mentioned above are very hard to dominate as a broad niche you need to find your own direction inside it. Think about natural traffic topics, if you your niche will natural search volume that it's probably have high paying keyword.