Grasshopper, the greenest rental car is one that is never rented. This is my sage opinion. Besides, haven't you ever heard of public transportation? Just about every major airport in the country is connected to a subway, light rail, or bus systemor all three. If you want to be as environmentally friendly as possible, opt for this route.
But if you absolutely must know what the greenest rental car is, I'd say it's one of the diesel Volkswagons and Jeeps offered by Bio-Beetle in Maui. The 100-percent biodiesel fuel that's pumped into them comes from waste vegetable oil, and the company uses only earth-conscious coolant and synthetic motor oil under the hood. The cost per day starts around $50.
In Reykjavik, you can rent a hydrogen-powered car from Hertz, but it's extremely pricey, and there are only a few available. If, on the off chance, you're traveling to somewhere besides Hawaii or Iceland, most of the major rental car companies now offer Toyota Priuses and other hybrids--but from my experience they're almost impossible to actually get due to the relatively low supply and huge demand. Your best option might simply be to forgo the luxury sedan and choose the high-mileage, low-cost compact instead. If you decide not to take the train instead, that is.