Gain Original Fresh HE
Gain Original Fresh HE comes in powder form only. It's rated well in Consumer Reports for its cleaning efficiency, cost and scent. It costs about $0.15 per load.
Some HE washing machine manufacturers will actually void your machine's warranty if you use non-HE detergent. Oversudsing can lead to a residue buildup in the machine -- specifically the rubber gasket in the interior door. This residue can eventually lead to mold growth and odor, so it's important to use the right detergent for your machine.