Out of all my experience in using linux I would say:
Ubuntu or Mandriva, for beginners.
Slackware or Debian, for Intermediate.
Linux From Scratch, for anyone who truly wishes to learn Linux, this can be beginners to intermediate as they supply an excellent manual for creating your linux.
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I use Slackware, as not only do I find it to be the fastest but the most reliable. It has my two favorable desktops, Fluxbox & KDE. The KDE which Slackware comes with seems like it would take a lot of computer resources, yet doesn't by its standard settings, you can turn all its flashy gizmos up and it does... but they are nice =D
You really do need to know something about linux to use Slackware, as you will find their aren't as many utilities for setting things up installed straight out of the box. I find it handy to have a laptop or something at hand when installing on my PC just encase I can't find how to setup the internet

just encase you decide to go with this remember this command "startx" it will start the xserver as it boots up into shell.
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The last version of Debian I used you have to apt-get the Xorg server yourself and by Xorg I mean XFree86 that how long ago I'm going back. This is what Ubuntu is based off, if you want a quick install version of this I would use Knoppix it installs the main things including xserver. First linux I ever used which was when I was 6 which I'm 20 now so its goin back quite a while.
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Ubuntu, I loved first linux which started working with my WiFi, but after a while it started to annoy me how slow it was, they have installed so much cr*p I would go nuts lol, also insecure, plus I hate the sudo command.
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Mandriva "Mandrake" was nice has always been nice, no problems with it except how slow it is, nicely polished but mneh its so nice it makes me sick, and its also slow on older machines.
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I would recommend anyone who has the internet and unlimited download, to install a Virtual Machine, and just try them out on that before actually installing them to your system =/ and if you try linux and don't like it, always keep a LiveCD version handy for any repairs to your machine. For a liveCD I would recommend BackTrack/