If you're just interested in downloading stuff, the only thing you really need to know is that WinMX is perfect for music, at least for now, because its full of high-quality mp3s (192kbps+), and has the ability to search specifically for OGG files and several others. I say for now because as of yet there are very few fake (static) files on the network. In fact, I have not even found one yet. I am sure they are there, but luckily I haven't ran across one yet... unlike with kazaa where I would jump for joy when I managed to find even one real file.
However. WinMX is terrible for videos, software, documents... in fact, anything BUT music. The queue lines are seemingly years long. Also, you may not have much luck finding somewhat uncommon songs on WinMX. However, if they are uncommon you should be able to find them on kazaa without having to sift through fakes, because the RIAA and music producers won't be worrying about them.
And of course, stick with kazaa for videos and anything but music. I still use kazaa lite 2.4.3 and it works fine.
As for your original question: K-lite 2.4.3 is the latest build from the people putting out K-Lite before they were shut down. 2.6 is some different people doing the same thing the people who were shut down were doing. Which means it is still illegal (hacked Kazaa Media Desktop code), just newer. Ressurection is analogous to that as well. DietK and CleanKMD are lesser known alternatives to the old Kazaa Lite. They all use the same networks, and are all hacked code and therefore all illegal. As for which is best, I attempted to answer that and give a little extra advice at the top. Hope this proved helpful. B)