My fav: modified Knoppix. So far I've used 3.1,3.2,3.3 & 3.4.
Next best: modified Knoppix. So far I've tried....
I've best fiddling with Linux about a year and a half. Had quite a time getting started, first with Mandrake (got it to download, wouldn't load), then Debian (some problems trying to get it to install too. Then got a 'Freeduc' disk (a Knoppix derivative) from Linux Format magazine. Wow, it actually just installed & ran, big change from Debian, no tons of setup which I didn't know how to do, but it had 'only' XFCE for a desktop & I wanted KDE so I became a Knoppix fan. No slur on xfce intended, I just like KDE more, especially really impressed with konqueror. Also got used to apt-get, nothing else even comes close. The only negative was (and still is) that I customize quite a bit on initial setup so upgrading from version to version is quite a chore. Thus I'm reluctant to try 3.5,6 or 7 unless there appears to be some significant improvement not just minor changes.
From the same magazine I've also tried Yoper (wouldn't load), Slackware (needs lots of setup & no apt-get) , Fedora core, a SuSE live CD demo and Mandrake 9.2. Mandrake was impressive in that it just installed and EVERTHING worked, even my scanner but I didn't like the way it Mandraked everything (setupDrake this and menuDrake that etc). About a month later KDE 3.2 came out & it took another month to successfully accomplish the upgrade which I was only able to do after somebody more skilled than me posted the mdk-rpm's. Even then, it was necessary to install whole mega packages ie, to get kpat you had to take all of kgegames package. and, with MDK 10 out, nobody is making MDK 9 rpm's any more, this is not a problem with Debian, a .deb is a .deb is a .deb.
So Knoppix it is because it's the easiest way I know to get a Debian system complete with a backup run from CD. If I REALLY had to pick a second (non Debian) I would probably use Fedore just because it's generic redhat.