FreeBSD is my preferred OS thanks to ZFS, disk encryption, the wholly-owned base system, ports/packages, jails for securing network services, and the community.
A few years ago, I found myself looking for a new OS for day-to-day use. I'd been using Mac OS X for several years because my employer provided a MacBook Pro and I really loved the nice, clean Aqua UI on top of a POSIX compliant core. However, I was setting up a desktop, didn't want to spend a fortune on Apple hardware, and wasn't sure I liked the direction Apple was heading with OS X by making it more like iOS. My requirements pretty much boiled down to security (including support for full-disk encryption), stability, flexibility, software freedom, and a thriving community for those inevitable times when I would need some wisdom. If you're reading this blog, I probably don't need to tell you why Windows was ruled out before the competition started.