I'm not familiar with FreeBSD and DragonFly, but I have used NetBSD in the past and a bit of Arch Linux. The system management is way more consistent in OpenBSD, things generally work and are more reliable. The package management system is a pleasure to work with, and when you want to remove unused packages or dependencies of previously installed packages, it's simple and consistent. It actually works. When you are configuring something, most of the time there is one single way to do it, and it's well documented. And the simplicity can't really be compared to Arch Linux. Fire up a vm and install OpenBSD to it, just for the experience. It's mostly just accepting the defaults, extremely simple.
