Windows 2000 was the peak of practical and usable GUI design. It all went downhill from there.
I recently went back and installed 2000 to make sure I wasn't forgetting something bad for usability in the UI about 2000 that others remember. Nope. I then installed XP to be sure with today's eyes it wasn't really better than 2000 and I just didn't realize it. Nope.
I didn't continue the experiment after that. I very clearly remember 7, since I still have to use it occasionally.
I see 2000s UI as being from a time when (for the most part) they weren't trying to impress you with the gloss on the UI. They were just trying to make it clear and usable. More modern iterations of Windows has some advancements in interactions but not much in the GUI controls themselves.
Me three! Windows 2000 is the best not just for usability, but still looks the best. No, all those fluff in XP and 7 don't make them look good, it make them look busy and dirty.
We may be in the minority, but only because relatively few people had a chance to use 2000 before it was replaced by XP one short year later. In contrast, people remember XP and 7 fondly because they kept using them for years and years since the subsequent releases, Vista and 8, were universally hated and panned.
Starting with XP more attention was paid to aesthetics and less to other qualities. E. g. XP controls consumed more screen space, which was a visible regression at times when 15-inch displays were still common.
Up until Windows 7 there was a classic desktop theme which was quite close to 2000 and I used it. Unfortunately Windows 10 has no such option. I use mostly FreeBSD and MacOS nowadays, but still have an old notebook with Windows 7 which I occasionally use. If I disliked Windows 10 less I would have upgraded it to Windows 10.