Lessee... Assuming you don't have a "get out of jail free" card like a PLB or working cellphone. In order of priority, each subsequent item assumes you have all the previous ones:

1. Knife
2. Firestarter (Bic or ferro rod)
3. Metal cooking container (purify water and cook)
4. Poncho/liner (mobile shelter)
5. Stainless steel wire (lashing, sewing, snares, fishhooks)
6. Canteen/water bottle (water storage & transport)
7. Paracord (infinite uses)
8. ...

If only one item, I would have to think hard about knife vs. firestarter. In winter I would lean heavily towards the firestarter, in summer I'd almost certainly want the knife.