Emissions are a major factor in the American Council for an Energy Efficient Economy's "Green Car" rankings, which the group has published annually since 1998. That's why the Civic GX tops the list despite fuel economy figures that are lower than the others. It runs on compressed natural, which is much cleaner burning than gasoline.
Emissions are why you won't find any diesel-powered cars on this list. Despite being very efficient, and much cleaner than they were in the past, diesel engines still emit more pollution than the most fuel-efficient gasoline cars.