Well Windows 7 is best in terms of security but most popular nowadays is Windows 10 and is mostly used by people.
I haven't wanted to update to Windows 10. I have no confirmation that some of the preliminary bugs have been fixed. I was on the insider and developer program but didn't install the pre releases. I want to back up my laptop's hard drive first and am currently in the process of mounting a 500GB VHD from my external hard drive in order to achieve this.