I am clubbing these two projects together as they have an interesting history. Mandriva Free was the very first Linux distro that I installed on my system, ever. Back in 2005, when Ubuntu was still cutting its teeth, Mandriva was the 'Ubuntu' of the Linux world. It was one of the most polished, designed and 'work-out-of-the-box' OSes and was backed by an established company.
Then Ubuntu happened and all such distros got pushed to the margins. Then the company became unstable and it led to the fork of the OS called Mageia. Then Mandriva decided to work closely with the Mageia community as most developers moved to the Mageia project after being laid off. Later, Mandriva become an enterprise OS and they announced a fork called Open Mandriva that was governed by the non-profit organization OpenMandriva.
Both Open Mandriva and Mageia are rpm-based KDE distros that offer a very pleasant and distinct user experience - totally different from what you see in Kubuntu-based distributions.
Mageia and Mandriva both have a neat control center, similar to but not as feature-rich as openSUSE's Yast. They both offer a great KDE experience, but both distros are going through challenging times, and similar to PCLinuxOS they deserve a mention on the list for historical reasons and also due to the fact that Mandriva used to be a great distribution that could have been what Ubuntu has become today.
From among the two, OpenMandriva is a more polished version that uses 'Homerun' for an application launcher instead of the traditional Kicker menu.
Had there been no uncertainty around Open Mandriva and Mageia, considering their polish I would have rated them as the top KDE distro.