Depends on whether you want a production server or just to have fun.

Knoppix will do everything you want, but isn't the most secure or stable distro (or, rather, the version(s) of Debian you end up with are not the most stable or secure). See the security thread for more details.

If you want something more stable (and all the security fixes) you can "downgrade" Knoppix/Debian (there's a thread somewhere here on it).

Slack is (probably) a good way of finding out about Linux, but it'd be a slog for a newbie.

Fedora (aka RedHat) is a common choice for servers.