Objectively speaking, I'd say Windows has, overall, continued to improve with successive releases. In terms of engineering, presentation, ease of use, security, and general usefulness, Windows 11 is the best yet. It's a very polished OS to use.
But on a personal level, I hate it - not because it's poorly made, because it isn't. It's the user-hostile design decisions like making MSAs mandatory, dark patterns that railroad users into doing things that help Microsoft (widgets always opening in Edge and not respecting your default browser, bundling ads and crapware in default installations, malware-like bait-and-switch tactics used to get users to upgrade, updates that can't be disabled) that really irritate me. Somewhere, under all that corporate policy, is a fantastic OS.
And I don't know if I'm the only one on the planet that's bothered by this, but I despise the way recent versions of Windows talk to me.
Around the time Office 2013 released, the UX teams at Microsoft took it upon themselves to start going crazy with 'informalising' the app language, turning phrases like "The file is in use by another user" into "There's someone else here", "Do you want to save your changes to xx?" into "Want to save your changes to xx?".
Windows 11 is full of phrases like, "Hi. We're glad you're here." "Getting everything ready for you, just a little longer." "[App] just got installed, check it out." Windows' UX designers seem to think that Windows should be talking to its users like it wants to be their friend, but it somehow just comes across as condescending.
Anyway, for favourites...
Nostalgia points me to Windows 95, as it was the first version of Windows that I ever used, and it was my introduction to computers - I like to fire it up in DosBOX every now and then just to relive the childhood memories.
I also have a soft spot for Vista - I got caught up in all the Longhorn hype and I find the development story behind it to be fascinating. The end product, after service packs were issued and hardware vendors caught up with its new driver models, was quite pleasant to use for the time. DWM and WPF were massive steps up in improving the user experience of the OS, catching it up to Mac OS X. The entire thing just looked so revolutionary and futuristic.
I also loved Windows 8... on a tablet. I know there was a horrible duality to it that just didn't make sense for either tablets or desktop computers, but the tablet experience, on a base level, was wonderful. I was sad that so much of the good work that was done in Windows 8.x was thrown away for Windows 10. Perhaps for the best overall, but Windows 10 didn't care about UX like Windows 8 did. There are so many nice details about the movement and arrangement of Windows 8's tablet experience that they just tossed away afterwards.