Well, there are no shortcuts; you just have to do Niche research. Plus, there is nothing as best niche for AdSense! It all depends on your content and Keyword research. I guess your question should be: “How to do Niche Research”.
Yes, health and fitness are saturated, but if you have a unique approach, you will always succeed. As for your question, I would say, whichever niche you settle in, if you want to monetize by AdSense, all you need is to have high-quality informational posts….I repeat….HIGH QUALITY!