...Windows 7, since that is in extended support already. Software in extended support only receives security fixes, and Windows 7 will continue to do so until 2020. In principle it means that Windows 8.1 users might miss out. That operating system is in mainstream support until January 2018, and during mainstream support Microsoft can potentially deliver feature improvements and other non-security updates. In practice, this is unlikely; Windows 10 is the only operating system receiving any meaningful feature development now, with Windows 8.1 only likely to receive security fixes from now until its end of support in 2023 anyway.