Just because I'm feeling particularly pedantic tonight: the OP asked for a window manager, not a DE
Ok, with that out of the way, I would recommend kwin (KDE's window manager) on gentoo-sources -- which is to say I would recommend KDE on gentoo-sources. Though, I suppose if you were wierd, you could use kwin on another desktop environment.

KDE has treated me well in the past and I like some of the GUI themes and most of the technologies they have under the hood, though I could do without some of the oversized menus

.
But like others have said, it is all about your particular tastes. If you don't want to install 80 bajillion window managers or desktop environments just to try them out, you could check out some live-CDs like Knoppix, Gnoppix, Ubuntu, or Mandriva.
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"The joy from having 3D flying toasters across two monitors almost brought a tear to my eye

" -- Me