I am mainly using XP. I have a laptop with 10 that has ssen more use lately, and a desktop with 7 that I built six years ago but barely have broken in. Instead I'm using this 12 year old XP machine which has all the software I need.
I still keep XP around, too. For certain specialized tasks and for certain older pieces of hardware, none of the never versions will do, and I appreciate its small memory footprint.