It's really up to you to decide, what you like best!
I prefer Debian, because it's 100% free software, and everything works!
KNOPPIX is based on Debian (the unstable version (sid) I think).
I started using GNU/Linux with SuSE 8.0 (because it was available to me), but I soon ran my head against a wall. I couldn't find the configuration-files I needed (non-standard!!!), and there was simple too much of my supported hardware, that just didn't work!
So I switched to Debian, everything works, and the installation wasn't bad at all (it isn't GUI but it gets the job done a lot better than SuSE's YAST2 - in my opinion).
Further more, SuSE has a tradition to include proprietary software in they're distribution, which makes it a non-free distribution!!!
If you are new to Linux and fear the command-line (please don't fear it, it's more powerful than any GUI out there - but alright, we must all start somewhere), I suggest you try out Mandrake Linux
, it's all GUI and very good. And the download-edition is free software.
But otherwise try out Debian GNU/Linux
Carsten B. Jensen
Edit: made little mistake with the software-thing of SuSE Linux