Joomla! 1.5
Joomla! 1.5 has improvements for all user categories. For clients that use only frontend to update articles and add news and links. For customers that use mostly backend to administrate whole site. For designers that use backend to edit layout. For developers that creates their own extensions. From my point of view best improvements are clearer administrator interface, easier picture management and overwriting files. Earlier adding pictures to media manager and beside text was pretty confusing. Overwriting files is a good improvement because it makes updating Joomla! much easier and faster.
Read more about Joomla! 1.5 from here!