Despite my avatar, I HAVE used GNU/Linux. I've used Debian, SuSE and Gentoo. Gentoo is my absolute FAVORITE distro with Debian being #2 and
SuSE being garbage. The only drawback is that the install is a bit difficult, but the Gentoo site provides a GREAT
installation guide for many different architectures (x86, PowerPC, etc.).
Imo Mandrake is the easiest to use straight up and good for a learning curve. Once you've got the hang of it switch to kubuntu (ubuntu with kde gui
instead of gnome), its very nice indeed
I usually recommend Fedora Core or SUSE for those new to linux, and for those who are past the gui hand-holding phase Slackware to customize into an
amazing server. It might be worth it to try out some live-boot disks first to figure out which distro or features suit you better so you can pick a
full install better.
I recommend Gentoo also. Its very customizable, letting you choose everything you want. it doesnt even come with a windowing system by default. Even
tho it seems like it'd be the hardest, since youd have to install everything and get it all setup yourself, I found it alot easier than the other
distros Iv tried.