Well I've only used KDE, Gnome, and Fluxbox.
KDE has a ton of features but is rather bloated. KDE's goal is for everything to "just work", and they execute that pretty well. To run KDE well you'll need a lot of RAM.
Gnome is also full-featured though a little more lean than KDE. You'll still need a sizable amount of RAM to run Gnome.
My personal favorite is Fluxbox, which is lean, very fast, and doesn't eat a lot of RAM. It's also highly configurable, though you'll need to edit some text files.
That's my 2 cents worth
Good luck picking the best one for what you want.