The Unix/Unix-like operating systems I've used:
SunOS, Max OS X (server and desktop), SUSE, openSUSE, RedHat, Fedora, CentOS, FreeBSD, NetBSD, gentoo, Debian, Ubuntu (all official variants plus fluxbuntu, but all are Ubuntu underneath), Knoppix, Puppy, DSL, Elive, Ximian, Arch, Android, Android w/Debian chroot, hand-rolled AOSP builds, Chrome, and various BusyBox/Linux builds (and DOS & FreeDOS, though they are nothing of a Unix).
My advice to newcomers: just use Ubuntu.
Of all of the above, I prefer to use Debian-derived operating systems, and I install GNU/Linux on all of my desktops, laptops, and servers. I love Ubuntu for its package management: core packages are updated every six months (on releases), LTS ensures stability when you need it, and PPAs are seamless.