Kubuntu, for the past 4 years.
Go Linux and you can never go back.
With Windows or Mac, you get to use a PC.
With Linux, you own it.
Hell yeah! 
I'ḿ surprised nobody else chimed in, though. I'd have thought, considering the obvious security differences, a lot more people would be using one Linux flavor or another...
Kubuntu I never used. I started with Ubuntu 9.10 (that is,
YEARS before Unity.

), and went straight for Crunchbang!. I have a soft spot for the Ubuntu universe, as it showed me the way into a world I couldn't have dreamed about a minute before. But once you get bit by the Openbox bug, it's hard to go back.