Depends on what it is you are after. If it is straight adsense, sure you could go after the highest paying BUT better be ready to compete against a TON of top notch seo guys and gals all competing for them as well. My suggestion, Locate smaller niches that pay off well in revenue. There is an endless supply of niches that pay decent and have next to no HARD competition. Write down a list of 10 to 20 things that YOU enjoy and then cross reference it with the google keywords CPC tool. You will find plenty of gems worth targeting.
mesothelioma (spelling?) pays about $30-$40/click. Good luck ranking for it though. And even if you do...good luck keeping an adsense account active for it. They ban people left and right for abusing that one.
Not sure there is a right or wrong answer to this one. But my first instinct would be the one that people are looking for the most and are ready to spend some money once they find what they want. If you get a good one and you have what people are looking for on your site and the purchase process is relatively easy You got GOLD!
Another good one is localized real estate. If you can get traffic for "cityname real estate" you'll make anywhere from $2 to $10 per click on adsense ads on your site. A friend of mine had a Georgia real estate site a few years ago. It would rank for just about every city, including "atlanta real estate". Before his site got banned (he did black hat), he was averaging $6/click. Unfortunately the real estate market is completely flooded online now and it's really hard to break into unless you really know what you are doing.
- mortgage - loan - finance - mesothelioma - lawyer are some the of the examples. You can get detailed key phrases by searching above keywords on https://adwords.google.com/select/KeywordToolExternal