Picking a genre isn't so much you thinking about genres and deciding what you want to write. Most of the time, the genre picks you. To butcher Harry Potter, the genre chooses the writer. Usually, you end up writing what you most read, because that's what you most love, and also, without realising it, you've already put thousands of hours of research in just by reading that genre.
I'm not here to tell you which genre to write in. The beauty of genre is that there's no right or wrong. Just write whatever you love to write.
When you've picked your genre, you have to pay attention to who your readers are. If you write crime for example, your readers are likely to be different from those who read romance. Once you know who your readers are, learn what they do and don't like about the books they read. Goodreads is a great place to discover this. If your readers aren't fans of swearing, don't swear. If they love an action scene, make sure to include some great action along the way. If they're all about dialogue, don't bore them with two pages of description.
With each genre there are certain expectations. We need to find out who did it; the two main characters have to end up together; the aliens must be conquered; we must learn something. What's expected of you depends on your genre. And once you know what your readers expect, you'll know which rules you can break. Breaking the rules and giving your readers something they don't expect can leave you with a book everyone's talking about. However, break the wrong rules and people will be dissatisfied.
Now, if you're looking for a challenge and want to grow as a writer, I suggest writing in a genre totally new to you. Don't worry, you don't have to show anyone! Changing genre every once in a while can really keep you on your toes as a writer. If you throw yourself into a challenge like this, your writing will improve in this new genre, as well as in your trusty favourite genre. You get to a point in your writing where you feel like you've stopped improving, and this is when you need to do some scary experiments to keep your skill growing. It's the same with anything. Once it gets easy, make yourself work harder!
Genre is personal and nobody can tell you what to write. However, switching it up every once in a while will help you push your writing to the next level.