It doesn't really matter what subject you choose though, not everyone will like your subject matter, even if it's a popular theme. There are as many tastes as there are types of artists, so the important thing to remember is to pick a subject you are passionate about and create it to the best of your ability. And most important, give it your own flavor.
Some subjects have been done over and over... ballerinas and children playing in the sand are two that come into mind. These themes do seem to be people pleasers, but how do you make yours so that it will be the one chosen from many?
Well, you can give it a fresh twist by focusing on one aspect. It can be a certain mood color, or unusual perspective or focal area. Size, composition, lighting are all areas you can play with.
You can take a clue from gallery work...gallery owners want several things in the work they offer their clients. One thing that is important is a recognizable style...a consistency in your work. Another is production, they work to build up a demand for an artist's product and while rarity may be a good thing, paucity is not. Certain subjects may be more popular than others but few collectors will want a repetitive subject on their wall (although they may want a lot of pieces from one artist whose work appeals to them).
The upshot is that you have to find a way to take whatever subject interests you and make it stand out in the crowd.