I too see Windows 2000 as Microsoft's peak for usability, at least for the GUI controls it supported. We seem to be in the minority though. You hear far more people praise XP or 7 as the best.

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.