If you want to experience the
real Unix feeling
then I would advise to pick one of the BSDs (FreeBSD, OpenBSD, NetBSD)
because they originally derived from
the ancestoral AT&T/BSD Unix code
(though completely rewritten by now for legal issues).
But at the command line and the way of how administrative work is done
that is pretty close to, well Unix
(of course today there are plenty of Unices, also there used to be a more remarkable
divide between BSD vs. SysV style which today somewhat has leveled out)
If you however would like to stick to a Linux distro
(because of the far larger developers' base the support for newer HW still is usually better, but on the other hand maybe also buggier?)
then pick up one of the
hard core distros like LFS (Linux from Scratch),
Gentoo, Arch, Slackware (and skip the installation of X
)