This is a great question, one that more people should ask about their appliances.
While the Energy Star label used to be the standard for energy efficiency, it has unfortunately not kept up with technology as far as their standards for efficiency. In fact I challenge you to find a name brand new model washer that is not Energy Star rated.
Remaining Moisture Content
When considering the efficiency of a washing machine water use is indeed a key variable. However, the evacuation of water during the spin cycle (a measure known as Remaining Moisture Content) is perhaps the most important measure.
This is due to the fact that most of the energy used doing laundry goes to the dryer.
The best way to reduce dryer load is to start with the lowest possible remaining moisture content.
MEF and WF
The best resource for researching all of these measures is the Consortium for energy Efficiency at http://www.cee1.org.
CEE is a consortium of unbiased parties who rate appliances on the most important measures of energy efficiency.
For clothes washers you will see two measures:
MEF = Modified Energy Factor. This is a combination of Energy Factor and Remaining Moisture Content. MEF measures energy consumption of the total laundry cycle (washing and drying). It indicates how many cubic feet of laundry can be washed and dried with one kWh of electricity; the higher the number, the greater the efficiency.
WF = Water Factor. This is the number of gallons needed for each cubic foot of laundry. A lower number indicates lower consumption and more efficient use of water.
Find the model that has the performance that you are looking for and then cross reference it with a consumer site such as Consumer Reports to see how it rates on value and durability.