Asking what is the best DE is like asking "How long is a string". It doesn't work standing alone. It depends on what you're used to and most importantly what you wish to do, how much you're wiling to learn, and on what hardware. Taste and personal preference should come later.
Also it is a moving target since WM/DEs change over time and in the case of KDE, sometimes rather radically. When KDE v4 first came out I would not and did not recommend it for beginners. It isn't only that it was more complex (but does more) but it was a bit buggy and really chewed resources. Sometime around v4.5.x that started coming together and getting smooth and more streamlined. However the full package still offers more than average users want or need. For one thing it is rich in development tools, only some (like Kompare) which have much use for the average user. It has educational tools like KStars which is a desktop planetarium that only some will appreciate. These are but examples of a full spectrum.
Gnome is more than adequate for most people and most boxes. Xfce works on just about everything and creates a pretty nice niche between austere and loaded. It's just kinda primitive looking even when tweaked hard. If the team ever gets really serious and/or funded I think E17 might blow Xfxe out of the water because it has a similar resource footprint and looks much more Big Pimpin'.
Bottom Line - if you have sufficient hard drive space, try 'em all or at least many. They're fun to check out and create a wide perspective useful in settling on one... or four.