There are those that will tell you that you should promote something that you know about or you are passionate about.
The problem with this is if the subject/niche is monetizable???
How many people are passionate about something that they find very hard to monetize?
I don't want to knock anyone's advice. I can just tell you from many many years of actual experience, that generating income from something you are highly knowledgeable or passionate about may not be very profitable.
The simple truth is that no one gets into internet marketing as a hobby, they do it to make money. Unless you want to lose a lot of money, which is crazy.
NOTHING beats lead generation, as long as you can do it cost effectively!!!!!!!!!