Well, of course, it won’t be just one type of light, you will also have a bare flash to play with. But if you can only have one modifier, I would say get a big octabox. But first, let’s define the octabox or as photographers fondly call her, the Octa.
The Octa is (usually) a fairly big source of light that has an octagonal front surface. Unlike a softbox for example, which has a square or rectangular surface. Of course, there are smaller octaboxs as well, but usually, when photographers want to use an Octa, they want that big semi-round light.