If I had to rank them, I'd put niche selection and topic choice first. A perfectly designed pin won't go viral if nobody is interested in the topic. The biggest Pinterest accounts seem to focus on subjects that people actively search for, save, and revisit over time.
After that, I'd say the image and headline work together. The pin has to stop the scroll and immediately communicate what value the user will get. Pinterest users decide very quickly whether something is worth clicking or saving.
Keywords are important too, but mostly for helping Pinterest understand where to show the pin. In my experience, keywords can improve distribution, but they usually can't rescue a weak idea.
The best performing pins I've seen tend to solve a specific problem, promise a clear outcome, or inspire an action. The design, title, and keywords all help, but the underlying topic is often what determines whether a pin has the potential to take off.