Every distro has it's purpose and target audience:
Redhat: Once the most popular distro, seems to be waning in popularity now. Has a crappy build environment.
Mandrake: Made for newbies. Everything is nice and pretty, but that's not always a good thing. It's OK for gettign a feel for the software available for Linux, bt you'll never master *NIX if everything is done for you.
SuSe: I have no experience with this distro, but a lot of people say it's great.
Debian: One of the two popular "hard" distros. Many say it's easier to use due to apt, but I could never figure out the initial package screen.
Slackware: The other popular "hard" distro. Modeled after BSD. My second favorite.
Gentoo: I don't know much about this distro, but everyone that has tried it seems to like it. I intend to take a look sometime.
LFS: My favorite. You get a bunch of source tarballs and a HOWTO, and are left to build and install the distro by hand. If you can build a running, useful system out of this, you are on your way to *NIX mastery.
There's also BSD, which is a *NIX derivative like Linux, but is not the same OS:
FreeBSD: One of the 3 popular BSD distros. Said to be easy to set up.
NetBSD: Wide-ranging hardware support. That's all I know.
OpenBSD: Security-centric. Initial setup looks cryptic but is actually easy if you have some idea of what you're doing and leads to a fully-functional firewall system in under 10 minutes. I like it.
Choose wisely and you shall go on to become an elite *NIX hacker. Choose foolishly and you shall once again be a slave of the Microsoft empire.