Oooh, the arguments will fly! ** My opinions only: **
Best beginner's Linux: Mandrake
Most hacker friendly Linux: Slackware
Most GNU friendly Linux (if you're Communist, Socialist or Richard Stallman): Debian
Linux most likely to take over the world whether we want it to or not: Red Hat
Now, my editorial comment.
I've been very unhappy with the Linux community lately -- I think it's become too bloated, too swollen and too ponderous. Also, there is a strong tendency to lash out at any kind of non-GPL development, even if it isn't commercial. I've gotten some nastygrams on a number of my (open source even!) works, saying I'm an idiot because I think GPL inadequately protects author's rights (this is a whole other topic). For a community that claims to be as inclusive as it is, Linux mongers can be worse than Microsoft apologists.
This doesn't matter to me anyway. While I'm happy that Linux has made desktop Unix a reality for many users (and Mac OS X will only further this), I'm a BSD user of long standing due to my University of California background, and almost all my servers are based on BSD (mostly NetBSD, although Mac OS X qualifies, and so does OS/MP [based on SunOS 4.x]).
If I may further plug NetBSD, it's open source, and its entire watchword is portability. I'm going to slap it on my spare Dreamcast soon, once I build the ISO and the cross-compiler tools on my Power Mac 7300. It also borrowed a lot of security code from OpenBSD, which is a very solid system, so it has good robustness against attacks to boot. It runs on Amigas, classic and Power Macs, PCs, HPs, Suns, DEC Alphas, SH3 based systems (including the DC), ... you name it.
http://www.netbsd.org/
Now that I've stirred the pot ...