Drupal 7 Most Popular Tags in a list as a block: How?
You can do this with views:


create a view of terms
add a required relationship to "content using tags"
set use aggregation: yes
add the "term name" field a second time, set one of the to "aggregation type: count"
add "term name" as a sort criteria, set "aggregation type: count"


A sample view is at http://drupalbin.com/19467