Wear a Helmet
No matter how careful you are, things beyond your control can conspire to pitch you down the road. When that happens, the only thing that will make a major difference in your future is whether you chose to wear a good helmet. The choice between no helmet (or a novelty beanie helmet, which is almost the same thing) and a basic DOT helmet can be the difference between "living" as a vegetable or having a normal life. A full-face helmet can further improve your odds, and if you spend time shopping, you can find one that is actually more comfortable than riding bareheaded. Although a jacket, boots, gloves, etc. can reduce your injuries, those are not likely to attenuate injuries of the life-changing sort, though a back protector could conceivably prevent a spinal injury. I'd say gloves, which might prevent grinding off part of a finger, are the second most important apparel items, but all protective gear is worth the price because it makes riding more comfortable.