1. Tarp - shelter, heat reflector, cordage, water still/collector/storage/carrier, raft's shell/sail, aerial signal, fish trap, shoes and clothes, wind screen, etc. The larger the better.

2. Knife - multitool. It must be a large knife.

3. Mirror - day signaling. Must be a large one. Other day signaling measures, including signal fires, require spending possibly limited resources.

4. Flashlight - night signaling, fire maker, night light. 4xD cells incandescent Maglite please. Modern D-cells have a tin shell and a lot of power, so with a knife you can strip a piece from one and short battery terminals with it to make fire many times. When depleted, the reflector can be tried for fire starting (depends on its type and size, those fascetted in LED lights are useless)

The actual list will depend on the actual items quality, your condition, weather, and location (which includes possible island resources and chances to be rescued), but 1 and 2 are essential, even if 3 and 4 are nothing at all.