Rankings must be used with extreme caution! The companies that compile rankings have a vested interest in making the rankings change each year. Asking “which is the best university in the world?” will give you only a subjective answer.
http://www.topuniversities.com/university-rankings
http://www.thecompleteuniversityguide.co.uk/league-tables/rankings
http://www.timeshighereducation.co.uk/world-university-rankings/
http://www.whatuni.com/degrees/home.html
http://www.guardian.co.uk/education/table/2011/may/17/university-league-table-2012