Water

Access to clean, drinkable water, for some of us, is so common place that it is, almost, unimaginable that we could ever be without it, but if you have ever lived through a disaster, like a hurricane, then you know that it is entirely possible.

In a survival situation, the "Rule of Threes" prioritizes what the body needs in order to survive. Normally, the rule states that we can survive

3 minutes without breathable air, and
3 days without water, and
3 weeks without food.
Some people even add shelter to the list: 3 hours (in a harsh environment) without shelter.
Although it is a generalization, it is still very useful in determining priorities in an emergency situation.

One of the best way to ensure access to clean, drinkable, water in an emergency situation is to have access to either a portable filter or a method of purifying water like purification tablets.