Arch Linux 2008 Overlord 64bit - The most 'real' Linux you will see because all it comes with is the 2.6 kernel & bash. Only add the software & XWMs you will actually use. Very fast.
SUSE 64bit Server - Because it comes with all the tools & security needed to make a good router/server. I still can't decide if this is better than Red Hat Server or not.
Backtrack 3 - Because it comes with excellent security tools & a clean KDE UI. Also works great as a HDD installation.
They are the Unix OSs I use....