Windows XP on my 2nd backup / classic gaming machine -- (Core2Duo, 2GB DDR2, Radeon X800 128MB)
Windows 7 on my backup machine / light gaming -- (Core2Quad, 8GB DDR2, Radeon 4830 512MB)
Windows 8 on my laptop (keeping it at 8) -- (A6-5200, 4GB)
Lubuntu 14.04 on my main machine -- (Phenom II X4, 8GB DDR3, GeForce GTX 960, 240GB SSD)
I've ran Windows 10 for a while but it felt too unwieldly with the autoupdates (which also sometimes revert users settings), random scans, randomly deleting files (basically anything it thinks might be malware), task scheduled stuff and other shit that goes on in the background. Turning on at random times at night and shit. Most of this I fixed, but that was all the "default" experience, and it seems like there's always something going on and the CPU usage is always higher than normal compared to 7/8.
If you asked me what my favorite OS is, I would say XP by far. It feels pure, simple, clean and responsive. Runs on just about any computer, and plays just about any classic game. And the 2D experience is to this day much faster than any of the 3D accelerated desktops of Vista and up. I've got a dedicated XP machine and also an XP guest through my Lubuntu host with VirtualBox. The 3D acceleration for games is actually pretty good and I can play most Windows games up to around Doom 3 flawlessly through the XP VM.
Going to try and stick with Lubuntu on my main machine since it has a nice XP like interface with LXDE with very low overhead so most of my system resources can go to my own tasks. Boots up ultra fast and only uses ~250MB ram sitting at the desktop. Very little random CPU usage. I might reconsider Windows 10 in the future with a platform upgrade, but seeing as I would then probably have to purchase a license, I might not.
When I think about Windows 10 for my own machine, I cringe. It has a mind of its own, let me do what I want to do.
When I think about Windows 10 for a relatives machine, i'm kind of relieved in some ways. It has a mind of its own, let it fix itself.