My opinion is somewhere between the above. I'm inclined to reject rpm based distros (mandriva, SuSE and Fedora/RedHat) essentially because there are nothing like the variety of packages available as Debian and because the package management tools are either wierd and idiosyncratic or essentially Debian rip offs.

This leaves Slackware, Gentoo and Debian. But all these are probably too much for a newbie.

This leaves the so called Debian based distros. I'd be inclined to reject Mepis, Ubuntu and Kubuntu because they don't use the real Debian repositories -- essentially they're bastardised distros which dilute Debian. This leaves Kanotix and Knoppix. Of the two, Kanotix is far more suitable for HD install, although I'm currrently running a 3.6 Knoppix installed to HD.

A Google for Kanotix will find you some excellent install instructions, the latest version and the forum (in both English and German).