In order of importance to me;
1. The stuff between the rider's ears, you know, that gray matter you could spill all over the road by doing stupid things. No amount of safety gear will overcome you doing something idiotic.
2. Brakes. High quality front and rear brakes are essential.
3. A good helmet.
4. Strong, high quality wheels, especially on the back where the weight is.
5. Good tires, where the rubber meets the road.
6. A mirror on the traffic (left here) side.
7. Lights and reflectors.
8. A horn or bell of some sort.
9. A well maintained bike. Loose fasteners could kill you.
There are situations where you are going to have an accident, no matter how good your equipment, awareness, and safe thinking are. Most motored bike accidents are cause by inattentive, stupid cagers who have no clue how dangerous a 2 ton chunk of metal can be. If you ride defensively, safely, and continuously aware of traffic and conditions you can reduce the number of unavoidable accidents, but being exposed and vulnerable is part of the experience.