I think Windows 9x was peak Windows.

I'd choose XP instead. People disputing the performance maybe should consider the hardware at that time. Real problem with 9x was low-level stability. Juggling with compatibility was difficult, file access comes to mind, it was a kludge. It was possible but hard to maintain the system in a sensible state.

XP was the first to bring NT architecture to desktop. It was a huge success. Many despised the colorful UI, I actually like it. They started moving things around, but annoyances were fixable. Microsoft has adopted more of a "my way or the highway" attitude since.

People also forget… XP had more than the one tonka-toy theme from Frog Design. There were several themes. Some appealed to my visual tastes, some didn’t… but I do think they were well designed, which is more than I can say for most UI design today.

Yes, maybe XP was the sweet spot between ease of use and ability to revert back to a "classic" Windows interface.

It also had many multimedia features for burning CDs, editing videos, etc..

XP also introduced activation, which is one of the reasons I prefer Windows 2000 to XP.

Wasn't 2000 the first NT architecture on desktop?

2000 is, by far, my favourite Windows OS.

XP was quite ugly though. 98 or whatever was the best looking, but 7 was probably best overall (because they’d at least somewhat improved the UI and the system was generally more stable and modern).

(FWIW I mostly switched to Linux after XP so this isn’t nostalgia).