From the story at the 1st URL below:
Japanese cars — three of them hybrids, one that runs on natural gas, and one a gasoline model — took the top five spots in the annual “greenest†vehicle ranking by an energy conservation group.

The gasoline-electric Honda Insight took the top spot in the list compiled by the American Council for an Energy Efficient Economy. The two-door was followed by Honda's natural gas version of its Civic GX, which took the top spot in 2005. Rounding out the top five were Toyota's Prius hybrid, the Honda Civic Hybrid, and the Toyota Corolla, which runs on gasoline...

Story: http://www.msnbc.msn.com/id/11276142/
Table: http://www.msnbc.msn.com/id/6969339/
More info: http://www.greenercars.com