Kubuntu
This distribution has KDE as it's default desktop.
- With a bit of configuration, it can be used by Windows or Macintosh users with very little training.
- It has a large community providing help to people who like to customize behavior.
Based on Debian with some help from http://ubuntu.com
Since the default desktop on Ubuntu, Disjointity, is unfriendly and onerous, very few people recommend using that distro.