1. Knife (probably a multitool in an urban environment, fixed blade in the wilds)
2. Firestarter (probably a good lighter, even a bic - that constant flame beats a ferro rod for me)
3. Flashlight of some kind
4. Shelter (poncho would be most versatile, heatsheet would be second choice)
5. Signalling kit (whistle etc)

Bonus things - metal cooking pot, waterbottle, first aid kit, cordage, cash etc.

But short term - shelter, flashlight and whistle are probably most vital