The Sustainable Cities Index was a report by Arcadis located here looking at the sustainability of the top 100 cities around the world!
To measure sustainability they looked at;
In essence they were looking at the social, environmental and economic sustainability of the city.
16 of the top 20 cities were European and the other 4 were Asian.
Here are the top 10..
If you are interested you can see the full report on https://www.arcadis.com/en/global/our-perspectives/sustainable-cities-index-2016/