I don't believe in the whole one item is all you need mentality, but rather what is the smallest survival tool kit that you could come up with.
In this case:
Life Straw - You need clean water to survive, so you need a filter of some sort
Knife - Cuts, cleans meat, cut stuff
First Aide kit - If you are expecting rescue within a few days or need to treat wounds immediately without the risk of infection you will need this
kerosene - start fires to stay worm
any addition supplies would be on the form of can food, MRE's, and Bottle water, if you can afford it buy a gun of sort to scare off looters while you wait for rescuers to arrive.