Mageia 1 - possibly the best KDE 4.6.x distribution
When I first heard about Mageia I was not too bothered, if anything it was just going to be another Mandriva. Buggy, random crashes, poor 64-bit support and useless.
But that didn't stop me from trying it and what an amazing find it has turned out to be. From the installer to desktop, everything just works. The installer itself is also quite impressive with enough options to keep power users happy but at the same time well balanced to provide a pleasant experience for newbies.
Going with the KDE 4.6.3 desktop, the experience is very nippy out of the box and in my view noticeably better than Kubuntu 11.04 > openSUSE 11.4 > Fedora 15 KDE.