1) Gaming
This niche was and will always be profitable because it's entertaining. Famous people like Pewdiepie made millions of dollars and still does just by doing what he likes.
2) Vlog
If you have an interesting life and you do a lot of different stuff from everyone else every day then this is a great niche for you. If you are just like normal persons and you do the same things every day then you should choose another niche since your videos would be boring.
3) Tutorials
This niche opens a lot of different ideas for new videos. Whether it is fixing cars, DIY projects, cooking or maybe makeup this kind of videos will always be popular because there are literally thousands of things you could teach someone how to do by creating a video.
4) Tech videos
Technology is evolving non-stop and new TV's, phones, computers and all kinds of machines are created every year. Before they are buying something most people want to see a review of that product whether it's just a phone or a car, and the first place where they are looking is Youtube because they know someone has already posted a video about that product. By creating a YouTube channel on tech niche you will always find enough content to create since, as I said technology is always evolving.
5) Fitness
More and more people start going to the gym but most of them don't have any knowledge about training or diet and buy a good trainer is pretty expensive these days. That's why people go to YouTube to search for pieces of advice and tutorials from persons who are willing to share it for free. You could make videos about how to complete exercises, how to create a complete workout, what food should you eat and many other subjects.
6) Style
This category is available for both men and women because everybody needs videos about how to dress for a special occasion, hairstyle tips, makeup videos, what accessories to use and the list can go on for a long time. During your videos, you could also review different products so you have another subject and also another way to make more money.
7) Food
This is a category where you can cover a ton of subjects because there surely exist thousands of receipts available, and thousands of ideas for videos about food so if you are passionate about food and cooking you can't fail with a channel based on it. Doesn't matter if you are a teen or an old grandma, you still need to cook and eat every day. Nowadays recipe books are not used anymore and people are using the internet to search for food information and recipes.