The most power efficient is going to be a system with an airside economizer that uses outside air directly rather than an air conditioned solution. Whether this is practical or not largely depends on you datacenter location. If the air you get from outside is hot, humid and polluted you may have to filter and de-humidify the air.
For more information see this White Paper done by Intel <a href=”http://www.intel.com/fit/pdf/Reducing_Data_Center_Cost_with_an_Air_Economizer.pdf”>reducing data center cost with an Air Economizer</a>.
If that is not practical then usually the next most efficient is some kind of closed loop cooling system that moves the chiller and air handlers very close to the actual heat source. As an example HP provide the <a href=”http://www.hp.com/go/mcs”>MCS system </a>which cools one or two racks and APC provides their in <a href=”http://www.apc.com/products/category.cfm?id=9″>row solutions</a>, other vendors will also have solutions that work in a similar way.
These all work in the same way, they by placing the chillers closer to the load you will use less energy to move air and by containing the hot and sometime cold air streams you reduce mixing increasing the efficiency of the cooling solution
This is what I am aiming for: Chillerless DC <a href=”http://www.datacenterknowledge.com/archives/2009/07/15/googles-chiller-less-data-center/”></a>