American Council for an Energy Efficient Economy
annual list of the most environmentally-friendly cars by class:
TOYOTA PRIUS
HONDA CIVIC HYBRID
SMART FORTWO CONVERTIBLE/COUPE 1.0L 3, auto stk
HONDA FIT 1.5L 4, auto
FORD ESCAPE HYBRID
HYUNDAI SONATA 2.4L 4, auto
SUBARU OUTBACK WAGON 2.5L 4, auto stk 4wd
NISSAN ROGUE 2.5L 4, auto
TOYOTA TACOMA 2.7L 4, auto
TOYOTA SIENNA 3.5L 6, auto
CHEVROLET TAHOE HYBRID C1500 6.0L 8, auto
NISSAN FRONTIER 4.0L 6, auto 4wd

full listing and reasoning of how they arrive at a green score greenercars.org

And vehicles with the lowest green score:
VOLKSWAGEN TOUAREG
BUGATTI VEYRON
MERCEDES-BENZ GL320 CD
JEEP GRAND CHEROKEE 3.0L 6, auto 4wd
MERCEDES-BENZ R320 CDI 3.0L 6, auto 4wd
LAMBORGHINI MURCIELAGO
MURCIELAGO ROADSTER 6.5L 12, manual 4wd
MERCEDES-BENZ ML320 CD
MERCEDES-BENZ G55 AMG 5.4L 8, auto 4wd
HUMMER H2 c 6.0L 8, auto 4wd
GMC YUKON 2500 c 6.0L 8, auto 4wd
BENTLEY AZURE 6.7L 8, auto stk
BENTLEY ARNAGE d 6.7L 8, auto stk