I personally prefer KDE. It is nice, polished and highly customizable (based on Qt). And the default configuration is very Windows-like, so it will be easy to adopt if that is where you are coming from. You can get KDE-packaged Linux distributions mainly as Kubuntu (based on Ubuntu), Fedora KDE Spin, OpenSUSE, Arch Linux and Linux Mint. I would recommend either Kubuntu or Fedora 17 (and, as of lately, I had a few bad experiences with the latest versions of Ubuntu-based distros (a bit unstable, buggy), so you might prefer Fedora 17).
At the end of the day, Linux desktop environments are a very subjective topic. You could ask ten different Linux users and get ten completely different answers. Everybody has their own preference, but there isn't necessarily a 'best' desktop environment per-se. It's more a case of finding the one that is best for you! So it ultimately depends on your preferences (how you want your OS to look and behave) and what works best on your hardware.
Gnome 3, KDE, Cinnamon and Unity tend to work more smoothly on newer hardware and these are probably the most modern, polished desktops available. For older hardware, there are more lightweight desktops like XFCE and LXDE. Then there are other, alternate Window managers that do not have a desktop per-se; like Openbox, Fluxbox etc. And then there are tiling window managers that allow you to control the entire desktop experience with the keyboard (so no need for the mouse) like RatPoison, DWM and XMonad.
Also, with Linux you can install as many different desktop environments/window managers as you like. So if you wanted to; after installing a Linux distro you could install several desktops and switch between them to see what they are like. Depending on the display manger/login manager your chosen distro uses, you can usually set which environment to use at the login screen (before logging in) by selecting the session type.
But seeing as you are new to Linux and may not be comfortable doing this; if you have enough bandwidth available, I'd recommend downloading and trying the LiveCD's from different distros using different desktops. So for example, you could try the various Debian/Ubuntu derivatives with their different desktops:
Ubuntu (Unity - Canonicals frontend for Gnome3), Kubuntu (KDE), Lubuntu (LXDE), Xubuntu (XFCE), Mint (Cinnamon - Another alternate frontend for Gnome3), Crunchbang (OpenBox), Debian (good old Gnome2).
Mint also has a respin available which uses Mate, which is a fork of Gnome 2 intended to continue development of the now defunct Gnome2.
Then of course for a plain, vanilla installation of Gnome3; the default Fedora liveCD usually comes with Gnome3 nowadays. There is also an OpenSuse respin LiveCD which uses Gnome3 (The default liveCD for Suse uses KDE, so you might have to do a bit of searching on the Suse site to find the Gnome liveCD!)
Either way, shop around a bit. Try a few different distros with a few different desktops and see which desktop you prefer!
Also, just as a heads-up: Most of the lightweight distros ship with equally lightweight programs (e.g Abiword and Gnumeric instead of Libre-Office), but there is nothing to stop you from installing and running more heavyweight programs on them! They will run just as well, if not better on a lightweight distro. So don't let a distros default set of packages put you off too much!
Personally, all of my machines contain slightly older hardware; so I currently use Arch Linux with Openbox and DWM. Openbox for when I want to use a windowed interface and DWM for when I'm using lots of text files. But that's just me!