I need to cms to manage a conference.
What is best free CMS for this?
Drupal is the most flexible open-source CMS that I know. It also has a Conference module that maybe does some of the things you need. The rest should be possible using Views/CCK (also Drupal modules).
Check out this Article about a Conference website created with Drupal, it is probably much more complicated than what you need but it shows what is possible with Drupal.