Chrome OS fave. Love my HP chromebook :D although, is not as good as my former Toshiba 2 chromebook.
I'm using Arch Linux and it's my favorite.
Just Windows 10 here, Im not sure what my favorite is ive probably yet to find out.
I'm using Windows 10. I don't have a favourite really!
Using OS X El Capitan. It is my favorite by far. I use Windows for gaming and some other Windows-exclusive programs, but I much prefer OS X (or now, macOS).
Windows 10. It's my computer's default.
My gaming PC still has Windows 7 on it as it's more compatible to some older games and takes the beta-test games better. My other PC has Windows 10 and they are both good set-ups in their own ways.