I chose a good leave-in, because that is the last product you put on your hair before you dry it, although in reality it's a combination of things. You have to use a quality shampoo (if you poo) and conditioner to get the right moisture, protein and porosity balance into your hair, as they do most of the work in keeping your hair healthy, but unfortunately, it is possible to
ruin all the benefits of those products if you slap the wrong leave in on your hair.
What I found helped me was to get the basics down first: find the right shampoo/conditioner combination that when you used them and you dried your hair
without using any other product, your hair came out soft, shiny and moisturized. Once you had those two products set, then you find the leave in and oil products that will help you prolong that nice, soft just-washed condition of your hair. For some, it's as simple as finishing with a light oil. For others (like me), it's a combination of a couple of products that work with my texture and porosity.
Good luck with your search!