While you asked for the one most important thing, here are many. The one most important for you is the one where you are currently lacking the most. 1. Being

While you asked for the one most important thing, here are many. The one most important for you is the one where you are currently lacking the most. 1. Being able to code well. This amongst others requires (a) Giving a high-level structure to your code, way more than what a programming language and tooling enforces, and (b) Being able to maintain a "stack" of what you are doing at any time as you think at different scales, down from what to name a variable, to the project goal. 2. Think of a project idea of your own. Whatever it is. And then make it. You may realize very shortly where the gaps are, in your own skills or in the state of the art in technology. Having a good knowledge of both, the possibilities and limitations leads to a better understanding of how to do what is practical at present. 3. Master presentation skills. These are useful not just in presentations, but in all communications. I recommend taking formal training or reading a book or two on this. Like everywhere else, training and coursework on this often misses the point itself. So pick carefully. 4. Be watchful of opportunities as well as politics at the workplace. This point may seem out of place or obvious, but it may end up being something of critical importance in your actual growth path. 5. Last but not the least, know what you want to do, though be always prepared to adapt to the changing industry needs.