WIn10 Disbeliever (me).
I have them all, old Inspiron 570 with Win7, my gaming PC (4790K with mild overclock) with Win8.1 and a few old laptops, which got free Win10 upgrades in the past.
All my Windows looks like Win7, because I use ClassicShell. Win10 has nothing to offer new to me (and possibly to a lot of people) except DX12. And I really hate Win10 for forced updates, especially for laptops.
Remember Anniversary update, which caused so many problems with laptops? Therefore, I completely disabled forced updates on my laptops, I revert this process once in a while or forcing certain update when absolutely necessary.
Win8.1 is promised to be supported till 2024, that is enough for me.
Regarding slow systems, My older laptop (previously Win7) is barely moving around with Win10, but it was OK with Win7, so I will vote for Win8.1 here, with both hands.
Anything else I missed?