Ubuntu at its core but uses the KDE Plasma desktop.
Ranked in these QuestionsQuestion Ranking
Pro Full featured desktop environment that is actively developed
Unlike some abandoned, unfinished distros, kubuntu is up to date and offers a fully functional desktop environment that targets workstation users.
Con Extremely unstable
There have been several bugs reported. Some of these bugs are: shutting down randomly, refusing to boot, refusing to shut down, menus may act strange, icons may disappear, etc...