Some of the many lists of think tanks are:
http://repository.upenn.edu/think_tanks/10/
The Global Go To Think Tank Index Report (GGTTTI) from the Think Tanks and Civil Society Program (TTCSP) at the University of Pennsylvania ranks think tanks worldwide. Categories used are
Top Think Tanks in the World
Top Think Tanks by Region
Top Think Tanks by Area of Research
Top Think Tanks by Special Achievement
http://www.thebestschools.org/features/most-influential-think-tanks
This list looks at the popularity of the think tank’s website, average yearly revenue, average printed media references (impact factor?) and the number of categories included in the Global Go To Think Tank Index. “It is based on the perceptions of hundreds of journalists, policymakers, and think tank employees.”
http://www.nira.or.jp/english/nwdtt/nwdtt.html
World Directory of Think Tanks
The World Directory of Think Tanks by the National Institute for Research Advancement (NIRA) – a Japanese organization. Provides a searchable database and lists by country.