Water potential is a measure of water's ability to conduct labor.
To be able to do some labor, an object must exert enough force on other things to cause displacement.
Water must be moving in order to displace another object.
The greatest amount of water potential that any volume of water has when standard atmospheric pressure is applied to that volume, which is defined as 0.
This is referred to as the water potential of distilled water.
As a result, distilled water has the greatest ability to move and hence displace other items.