It was a solid OS, limited mostly by lack of mainstream 64-bit version and hard capped at DirectX 9.0, meaning modern games no longer worked. Later on, lack of TRIM support for SSDs.
Basically between SP3 release in 2008 and XP's end of support in 2014, you could have used a very stable OS if you didn't need to play the latest games or run the absolute latest hardware.
That was only an issue with the DOS-bootstrapped versions. The NT line was a lot more reliable for early adopters. Or at least it was until XP came along. Which is another good reason why Microsoft peaked with Windows 2000.
I think that there were several different peaks. In terms of UI, I'd say Chicago is the peak. In terms of internals, you're probably right, it was 2000. In terms of performance, I'd say it was probably Windows 7? Maybe it was just because I actually put together my own tower instead of pre-built?