I would have to say Slackware or Debian.
Slackware is great because it forces you to learn how to do certain things the way it was meant to be done.
IMHO it also has the most logical file structure.
Currently I'm runnix Knoppix, which is based on Debian.
Also, I'd recommend you NOT to use any of the desktops (Gnome or KDE).
This is how it should look :-)
