Gnome, KDE, XFCE... which is most simple and customizable?

I know how you feel; I tried so many different distros before getting a feel for the differences, and I continue to try new ones, usually in a virtual machine or a spare partition.

I don't really find Gnome to be slow and bloated, but I'm not too happy with the direction it's gone recently with the Gnome 3 shell. Gnome is fairly simple compared to KDE, but not nearly as customizable, and it's getting less customizable, it seems.

I find KDE to be overkill, and I've never spent enough time to feel comfortable with it. It has lots of options, widgets, etc, and of course, some people love it for that. KDE may also have the most variety of (native) apps available, but I haven't used it enough to really know.

From your post, i think you should look at XFCE. It's the least bloated of the mostly full-featured environments, without a lot of extras. It will run most of the same things that Gnome runs, as it uses the same GTK toolkit, I think.

However, it's possible to run KDE apps using Gnome, etc, buy you need to also install whatever supporting libraries, etc are required for them. A lot of people don't like to do this, but if you have plenty of disk space, it's probably fine.

Being new, I'd recommend sticking to a mainstream distro like Fedora, Ubuntu, Debian, etc, as 3rd party software is more likely to have an easy install for their programs.

I should add that this is all a matter of opinion more than anything else, unless you add some more criteria that might differentiate the differences more.

Simplicity to use and configure often don't go hand in hand. Gnome is supposed to be easy to use, but it's pretty big and doesn't have much in the way of customizability. KDE is also big, and more customizable, but the customization options aren't always straightforward. Other alternatives include LXDE and XFCE, which are more lightweight than either Gnome and KDE, as well as not using a desktop environment.

One choice you'll have to make is whether you want to use a desktop environment or a window manager. A basic window manager only manages windows, and many provide additional services such as ways of starting applications and multiple workspaces. A desktop environment adds many more features, generally accessible through icons or other widgets on the screen in panels or docks (clock, network manager, email notifications, removable disk mounting, …), and integrates a session manager. Desktop environments give you more features; with a mere window manager, you have to build your desktop environment piece by piece.

The utmost in configurability comes from window managers that are scriptable in a programming language, such as Sawfish (Scheme) and Awesome (Lua). There are plenty of minimalist window managers, but they are not for the faint of heart.

You'll want to try out various possibilities and look for something that suits you. You can use a program like Xnest or VNC to try out a new environment inside a window that itself is displayed in an environment that you're more comfortable with.

Apart from a few smaller distributions that offer a limited selection of packages, the choice of distribution is irrelevant to the choice of graphical environment.

Awesome maybe worth looking at as well as it matches all your criteria. It should be available from your package manager. The awesome wiki has a plethora of examples.