It really depends on what YOU want. Do you want an "automatic" type distro that does everything (almost) for you, or do you want something that you can learn about Linux with?
While the auto distros aka newbie distros are easier to install and use initially, you tend to learn less about Linux using them. Mepis and Ubuntu are pretty popular.
Any of the 'newbie' distros will work if that's what you are looking for.
If you really want to learn about Linux, I'd suggest trying Slackware, Gentoo (long compile times), or Debian.
Slack is the most 'unix-like' of the distros. No automatic configuration tools, very stable and fast (assuming you configure correctly). Some third party package management tools, but better to do manually to avoid problems.
Gentoo. Source based distro, very stable and fast (assuming you configure correctly. Everything is compiled on your box so installation can take a long time (usually a couple of days). Fantastic documentation and package handling.
Debian. Very good package management. Huge software repositories, but apps are usually behind what's current. Solid distro.