1 in 8 sites online contain adult material. But to pick a topic just because it is popular is a weird strategy. If you are in this to make money you'd be much better off picking a niche you can really get interested in. And before you do anything for the purpose of making money you should write a plan about how you are going to make your business work. Build it and they will come simply doesn't work. Just starting up another health blog or make money blog with no real plan is a complete waste of your time.
I think it would be how to make money, get rich, health, website designing and others similar to those!!!