A CMS with the most extensions would not automatically be a mature and admin-friendly CMS. (And why are you interested in a "complex CMS"?)
What I would do is define of your requirements (both in terms of functionality, but also in aspects related to performance, platforms, enhanceability, flexibility, useability, -ility). After that, make a shortlist of candidate CMS'es. Then evaluate the CMSes and their extensions/add-ons against your requirements. Don't forgot important aspects as developer activity, user community, the 'culture' of the Open Source project.
Alfresco, Drupal, Joomla each have advantages in their own areas and bonus sweet spots. Researching those, and the fit with your requirements, is much more important than "the one with the most ...".