Linux

BSD UNIX is a great place to learn. Also the documentation is very good. FreeBSD is used in production environments.

Brian Kernighan's book is also a must for getting your bearings with the shell.

No Linux distribution is good for learning Unix. Note that I used the term "Unix" instead of "UNIX". There is a difference. UNIX is not an operating system but a branding that indicates that a conforming operating system meets the certification requirements of the UNIX brand which is administered by the Open Group (TOG). Unix is a class of operating system.

The reason that I say no Linux distribution is good for learning Unix is basically that Linux distributions use GNU shell, commands and utilities and tool chains such as GCC. These are generally not available OOTB on commercial off-the-shelve Unix operating systems such as Solaris, AIX and HP-UX.

I concur with the other posters who recommended Solaris 10.

I had the same sentiment a few years back when getting into *nix. I finally determined the best way to get into learning it was to avoid the bloated distros that offer 3 or 4+ of every flavored option out there so as to focus more on the underlying system. Slackware seemed like the best boiled-down distro, but I did want some GUI guidance and ended up going with zenwalk, which started as mostly a light weight desktop (xfce) integrated with slackware.

I don't know if I would have learned more or less with another distro/OS, but it has worked for me and kept me funtional without Windows all this time. Now however, I have a new notebook and am about to try out Arch as it has a similar minimalistic approach, but I saw indications of more of a focus on newer hardware (and hopefully less on graphical smilies)-:

Absolutely. GCC, the many available open shells, the GNU toolchain, and GNU utilities couldn't ever be mistaken for ground-up reimplementations of standard UNIX utilities. They don't even have the same names. </sarcasm>

Specifically GNU/Linux I have installed and ran Gentoo which has bsd like facilities. Though it has a very do it yourself feel which can be kinda fun. I have also done arch on a netbook. It has a bsd like rc.conf but the syntax is very different than freebsd's rc.conf.

Either way regardless which os you use taking the time to learn with this small list:
Know your basic posix commands and your system hier. Know how to start/stop services, compile and install your base, kernel, and userland. learn enough vi to configure your system files and configurations. Finally AUTOMATE EVERYTHING.