Arch, because I can never be happy except when I'm bickering with a machine.
Seriously, though, I like the control and the learning factor. I enjoy knowing what my computer is doing and why, AUR is great, and the documentation is generally top-notch. Once you get past the point in the learning curve where everything is on fire and you don't know why (don't forget the 'linux' package when you pacstrap, kids!), it's a delight to use