What cars are the most friendly to the environment?

What cars are the most friendly to the environment?
The Top Ten Greenest Cars. Source: ACEEE's Green BookRanked by Cleanest fuel emissions10: Hyundai Elantra9: Toyota Corolla/Matrix8: Mazda37: Honda Civic Hx6: Nissan Sentra5: Toyota Echo4: Honda Civic Hybrid3: Toyota Prius2: Honda InsightBest Vehicle 1: Honda Civic Gx (Compressed Natural Gas)