It depends on what you intend to do with Linux.
Do you just need a system for your everyday use at home, like playing media and surfing the net? Then go for one of the distros that are more beginner-friendly, like Mint, PCLinuxOS or Ubuntu.
Do you need a stable system for production use? Then go with Debian, Slackware or CentOS.
Do you want to learn as much about Linux that you can? Then go with Slackware, Arch or Gentoo.
Please note that these only are some suggestions, that are somewhat interchangeable. Of course you can set up Slackware, Debian or CentOS as multimedia system (and many here do), and you can learn about Linux on one of the newbie-friendly distros.
Only the stable production OS is somewhat limited to only a bunch of distros.