Water Purification Kit

No matter how much water storage you have, there is a chance you will run out. Make sure you can add to your stash if need be.

This is the best choice because water can be found everywhere. If world falls into a world of disaster, then there should still be some living creatures or plants which you can get liquid from (urine is one idea), but it is not suitable for human drinking, therefore purification kits can help a lot!

I think that is a good idea... Purifying water would be needed more then storing water... 
There were lost of cases where people had no water to drink and they purified
The water and survived, 
So, I think that its better to be ready with water purifiers then being prepared with gallons and tones of water...