NetBSD, OpenBSD, and FreeBSD. They are all fun. They are different than Linux. I am of course talking about the kernel rather than the GNU software that is typically available on all of them. The BSDs have a vintage feel about them. I like it.
I wish that it was easier to try System V or SVR4 rather. Because instead of the Berkeley line, it is from the AT&T line which was the basis for IRIX, the version of UNIX on SGI systems. I still have a fondness for the MIPS10000 Octane MXE workstation I had many years ago.
The only way to try System V nowadays is to use an OpenSolaris derived system. But I have to admit that I hated Sun systems when I was younger for no good reason. I think it was because the 3D graphics sucked and I do graphics! So, it left a sour taste. Though there is a Sun variant called Tribblex that looks like it could be interesting to try in a virtual machine.
Of course, the BSDs still feel like the best out feel out there. I want to set one up with a nice X11 desktop like MWM. And it would be nice if IBM and HP would release AIX and HP/UX as free software. And maybe SGI will release their source code for their X based desktop and life would be grand.
UNIX could become more available to the common folk. I was watching some classic Computer Chronicles about UNIX. In 1985, it was still crusty and untrustworthy. In 1989, their last chance to prove UNIX was worth using, all they could talk about was how you could use a menu and how there machines cost $5000. Lol.