Windows 8.
(yes, hardly anybody remembers that there was a Windows version between 7 and 10 - but it did exist, I'm not making it up, saw it with me own eyes on a coworker's PC once).
Nah, I still have my boxed Windows 8 Pro on my shelf. The key used to activate Windows 10, maybe even 11, but last I tried it didn't work anymore.
Considering Windows 7 (if you count ESM) was supported all the way up to 2020, it's no wonder people skipped 8 (start menu tiles aside). It was a weird release schedule, and it was split into two versions 8 and 8.1, with 8 only having like 3 years of main stream support, and no ESM and 10 released just two years after 8.1.
If you count paying for ESM, someone could have gone from XP->7->11 and still been within support the whole time. Or from vista straight to 10.
Fun fact: Windows for Workgroups 3.11 was supported all the way to 2008. I believe it was the longest supported version of Windows.