Asking questions like this usually leads to difficult and confusing answers, because everyone has their favourite distro which they will promote. But I'll give you my opinions for what it's worth:
A recent poll on a kde website (kde-apps.org I think) ended up with the result of SuSE being considered the best KDE setup from any distro. I use SuSE Proffessional 9.1 myself, and have been very happy.
Another distro I have been very impressed with is the latest version of Xandros. It's as close to a drop in replacement for Windows XP as I have seen, which is a big plus if that's what you are after.
I would suggest a strategy of downloading four or five of the most popular distros that are recommended, possibly as a "live cd" (that's a CD that will boot up the Linux operating system without having to copy anything to your harddrive -- most distros will make a live cd version available for download) and trying them out over a few weeks. Take it slowly, and decide, ultimately, based on your own testing and your own experiences.
For reference, try
www.distrowatch.com. They have a list of the most popular distros on the far right of the home page. Pick the top 5 or 10 (note: not all of them are KDE based distros) and give them a try.
Cheers,
L.