Debian Lenny/Sid
Awesome distro, very flexible, wicked packing system, is a core distro for a lot of other distros (Ubuntu for one).
Maybe the only downside I have had, is with my laptop. Lenny/Sid version of X took a while to fix a major bug, which made my fonts (every) extra large. There was a work around for it though, and it's since been fixed. Did take a long time though.
But overall 9/10 for the distro. I mainly use tty's for my work, but I do use fluxbox, xfce and gnome from time to time.
Why do I use Debian over any other distro? Because I haven't experimented with other distro's all that much yet. I've used maybe 5 other distros, but not for a long period of time, just quick glances, how there packing system works etc. For instance, Red Hat 9 (long long time ago), Gentoo etc. Plan on testing out a lot more distro's in the future, so my "frav" distro could well change.
PS, might be nice if you add a poll to this thread, see what's the most commonly used hak5 distro! :D