What do you think is the most important factor to succeed in creating your own startup?

As you all may know, when creating a startup the founder should focus on many aspects of his business. Such as idea validation, product development, marketing, customer relationship, hiring employees, and so on. Despite we all know that all of them are important if you had to choose one core aspect to focus on at the beginning to increase your chances of succeeding (or reduce your chances of failing) what would it be? In my opinion, it should be to validate the idea correctly in order to make something that people really want. But you may think that it is more important to have a perfect marketing strategy or a 10/10 product development. Whatever. Looking for your opinions about this!