To me that was Windows 2000. One day in 1999, I was at the local bookstore going over computer magazines and one of them came with CD to preview Windows 2000. I was mostly a Windows 98/RedHat user at that point, so I decided to try it out.

It almost instantly won me over with the leap in stability due to the NT kernel, but the craziest thing was this feature called "Hibernate". This was the time when booting was painfully slow, and here was a feature that not just booted rapidly, but dropped me into the previous session with all apps open! It was pure magic. I switched over to Linux exclusively a few years after that, but this was the feature that prolonged that decision for a long time. I don't think Linux ever got a useable hibernate, but the feature became not as necessary due to the advent of SSDs.

Windows 2000 was the last version where Dave Cutler ran the whole show.

There are certainly features in later versions I wouldn't want to live without, but the decay began when he was moved to other products.

Win7 desktops with W2008 backend was a solid platform with robust ecosystem

They blew it all up with Longhorn/Metro, and have been doubling-down on denial ever since.

windows xp pre uac was a golden age ;)

win98 SE

Peak usability by being able to type a url in to the file manager or a local path in to your browser and have it open in the same window.

Same in Nautilus.

and also peak crashibility. con/con, anyone?

I think you're mistaking Windows millennium for Windows 98 SE

I never got into ME. 98SE was my jam.

While we're down this rabbit hole, my favorite bit was wrapping up the versions of windows: CE, ME, and NT: Windows CEMENT.