It depends on your needs. Lithium and JiveX are the industry leaders and the most robust and flexible solutions for complex communities. They are costly here, but they can handle just about anything and do much of the work for you.
Other good software include Drupal, vBulletin, and Discourse. They are more affordable, but require you to have more tech know-how or have a team of developers to get it up and running properly and securely.
