I've tried almost all of them and Window Maker is my favorite.  It's fast, lightweight and iago likes it too.

It's also very functional, if you know how to use it well.  The only real downside is that it's not terribly pretty..

ew /me dislikes Window Maker

I voted Gnome, since I didn't see 'other' at first. I think my favorite is Fluxbox.

I prefer fvwm: also lightweight, and very configurable if you know what you're doing.  Can bind dozens of functions just to the buttons that decorate the edge of a window (but if you do, good luck remembering what all of them do! :)).

Window Manager?

I'm a fan of KDE and Fluxbox. Window Maker is good too. I switch between them often, depending on how bored I am. heh.