My favorite versioning system was Darcs for a long time, but it didn’t really took off and a new competitor is taking off amazingly fast: Git. I believe the single most important feature of Git is to be able to clone (checkout in Subversion jargon) repositories from other systems, particularly SVN. That means that your favorite project may be using SVN, but you just clone it and work with Git, in a distributed way, and then send back the changes. Or that you can clone the SVN-repo and basically you already migrated. Amazing!
The single most important feature of Git