This is a question that the jury is very divided on. Kubuntu in my opinion is not a particularly good distro. and has often fallen short by quite a long way of Ubuntu itself. My preference is Linux Mint 10 KDE. However it is not so long ago the KDE 4 Desktop Environment was horrendously unstable. I remember the hype surrounding the release of KDE 4 on OpenSUSE 11. It was without doubt most disappointing. OpenSUSE 11 had more than its share of bugs and coupled with the newly released KDE 4 rendered it pratically unsuable. To prevent the onslaught of OpenSUSE users I will add things have improved somewhat.
Linux Mint 10 KDE Download
http://www.linuxmint.com/edition.php?id=76
Linux Mint 10 KDE ReleaseNotes/User Guide
http://linuxmint.com/rel_julia_kde.php
Another KDE based distro. worth considering is Pardus Linux
http://distrowatch.com/table.php?distribution=pardus
If indeed Microsoft have copied KDE then they did it well. Even myself an avid Linux user will give credit where it is due
Moving on to GNOME I thoroughly recommend Linux Mint 9 LTS and Linux Mint 10 Main Edition which are built upon Ubuntu 10.04 LTS (Long Term Support) and Ubuntu 10.10
Linux Mint 9 LTS Download
http://www.linuxmint.com/edition.php?id=52
Linux Mint 10 Download
http://www.linuxmint.com/edition.php?id=67
Linux Mint 9 LTS Release Notes/User Guide
http://www.linuxmint.com/rel_isadora.php
Linux Mint 10 Release Notes/User Guide
http://linuxmint.com/rel_julia.php
Also worth considering is Fusion Linux 14 which is a Fedora based distro.
http://distrowatch.com/table.php?distribution=fusion
LUg.