The concept of urban greenness is not a strictly defined one, which makes it difficult for any organization, website or government to actually come out with a list of cities that are green and those that are not that eco-friendly.
For instance, one classification may rely mostly on biodiversity as a concept which shows how green a city is (number of different species living in a set area) while others might rely heavily on how widespread recycling is, how much water is wasted, how much people use public transport or any other criteria that may be put under the greenness umbrella.
Despite the lack of actual, official lists of green cities around the world, some organizations are trying to come with their own lists which may be flawed but which we have to take into consideration due to lack of aforementioned official ones. For instance, there is the Green City Index, sponsored by Siemens Group and put together by The Economist Intelligence Unit.
According to this index, San Francisco is the leader in greenness in this part of the world. Curitiba in Brazil and Singapore lead their regions. To give you an idea of what the index values greatly, we can tell you that Singapore ranked at the top due to its water reclamation policies.
Together with these three cities, Copenhagen also finds its spot among the greenest world cities. However, once again, we have to point out that the results might have been different if other criteria were valued more highly. In the future, we can hope to get more comprehensive and detailed lists.
Until then, kudos to San Fran!