Former Mayor Richard Daley made Chicago a leader in green roofs and buildings. New Mayor Rahm Emanuel will be expanding the budding bike network even more.

Cohen likes San Jose's Green Vision plan, its track record on setting environmental goals and tracking progress, and its abundance of cleantech investment.

Philadelphia has aggressive, transparent sustainability targets in areas such as energy, buildings, GHG reductions, waste, transit and agriculture.

Density, transit, and the climate leadership of Mayor Mike Bloomberg all scored big points for the Big Apple.

Sunny San Diego is ramping up its use of renewables, aiming to add 50 megawatts of renewable energy by 2013 while reducing demand by the same amount.

An ambitious light rail program -- 119 miles built in just a few years -- and a decade of proven leadership in the low carbon economy made Denver a strong contender.

The federal government may not be showing much leadership, but the District ranks among the top 3 cities in the nation on green buildings, transit use, and purchase of renewable energy.

Cohen quotes Portland's mission to “be the most sustainable city in the world by investing in high performance buildings and green streets, ecosystem restoration, businesses that create sustainable economic opportunities for all, green and healthy affordable housing, and social equity policies and practices.”

A major utility that is carbon neutral. A former mayor who launched the U.S. Mayors for Climate Protection. Universities and cleantech firms on the cutting edge. Add a passel of LEED-certified buildings and that's why Seattle comes in at No. 2.

Cohen says that San Francisco ranked in the top 3 in every category he measured. He chalks up the city's across-the-board dominance to political leadership and vision.