I use Win7 Ultimate SP1 now.And used to have Vista and Win XP Home Edition as it was easier to upgrade from Vista to Win7 a few years ago and still enjoy Win7 more than any other OS for stable and rarely find an error and i have a full file system storage on an External HD to rescue me in any time some bad thing happen.
I have been using Windows 8.1.1 but lately have switched to using Windows 10 Preview. Starting to get used to it and love it. I will not go as far as to say I will buy it in the final version. All depends on the cost.