Various organisations have ranked the world universities. The results varies in accordance to differences in the methodology they used. Shanghai Jiao Tong University listed Academic Ranking of World Universities 2006
The ranking was based on citations per paper as an indicator of quality of research. This is a simple method but it can provide general indication for researchers and for ones who look for good research-based universities. Why? Because research is the very basic identity for many world class universities -- off course we also cannot undermine the existence of professional universities but they are not comparable.
However, any ranking procedure using university level as unit of analysis cannot be used as a main guide to judge the quality of particular subject at certain university. It is common that each university is strong in certain parts and is weak in other sides. Therefore, someone -- someday and somehow -- probably shall rank faculties and research institutes associated with urban and regional studies (?).