I heard that medicine is the best niche, but I might be wrong. I'm starting a site soon and need some input.
Something you care about and don't mind putting time into to build a good resource for people... You'll make a lot more money doing that than chasing the latest "high paying keyword" niche that everybody else is chasing too. Plus, those high-paying keyword niches are usually boring as heck to write content on. Lemon lawyers in California, oy.
Easiest niche to start is something that you enjoy. The more you like it, the more you will write and spend time to develop it.
If you are going after medicine you need to be very specific and narrow it down to a thin slice. For instance....... Diabetic is big keyword. Diabetes in Men is narrower. Diabetes in Men over 40 is even better. I suppose you could even take it down another notch. You have to choose your niche sliver and not be general.
I have a post with the highest paying adsense keywords for September if you're interested: http://www.nethustlin.com/2007/09/highest-paying-google-adsense-for.html But as webcosmo said, a lot of these keywords are highly competitive so it'll be hard to rank in Google for most of them. My blog is about making money online and I usually get about .50-1.50 per click, which I think is pretty good. Anything that is slightly competitive will probably have a good return.
I doubt if Google will pay that kind of money to a publisher who builds site for the sole purpose of generating revenue from adsense.
Something you enjoy is extremly easy. Content is king! and .. blogs filled with videos are also easy money ttry that!