Solaris is cool, BUT: it is a huge pain in the butt to find, download, and apply patches. The GUI works kind of OK, but otherwise, you have to research on your own what patches you need.
My favorite OS of all time? OpenBSD (not FreeBSD). Why? In more than a decade, there have only been two remote security flaws with that operating system. I double-dog dare you to name another networkable OS that is that secure. Of course, the problem is that Adobe doesn't make Flash for any of the BSD operating systems (unless you count Mac, which is Darwin BSD). As much as I hate Flash, the problems it causes, and Adobe's ironclad control over the standard, I use it a lot. I need it.
FreeBSD has a Linux compatibility mode which allows you to use Flash for Linux, but the way I see it, if I'm running FreeBSD in Linux compatibility mode, then why not use Linux in the first place? Among the Linuxes, I think that Ubuntu is the best for workstations and end-users, while CentOS is best for servers.
Now onto the most "interesting" OS's. I'm torn between two:
ReactOS ( http://www.reactos.org/en/index.html ): while not perfect, they've made quite a lot of progress so far in building an OS from scratch that can run Windows software.
Menuet ( http://www.menuetos.net/ ): Also known as MeOS, this OS is not as feature rich as most others, but it's written ENTIRELY IN ASSEMBLY LANGUAGE. The result is that it's compact and very, very fast.