To which, people throw out things like forex, dating, weight loss, etc.
Got a clue for you, people. None of those niches are profitable. None. Zero.
Please hear me out before you call the guys in the white coats to come and take me away.
All of those markets, niches, and many, many more have characteristics that lend themselves to making a profit.
They have large enough numbers of people interested in them. Many of those interested have money and have demonstrated a willingness to spend it on that interest. All of those markets and niches within those markets have multiple ways to place an offer in front of the people ready to spend money.
But simply possessing those characteristics does not make them profitable.
Only you can make them profitable.
You have to connect with the humans in that marketplace, and make them an offer they're willing to give up money to accept. And you have to do it often enough, with a great enough margin, to reach your own financial goals. Once they accept your offer, you have to deliver enough value that they allow you to keep their money. Even better if they feel slightly guilty for getting the better end of the bargain.
For you, any niche where you can accomplish the above is a profitable niche. Any niche where you cannot, isn't.
Bottom line, it isn't the niche that's profitable. It's doing the right things to provide value to the people within the niche at the right price that's profitable.