I'm extremely nostalgic for 98 (was my first Windows OS), but I definitely think its tied between XP, 7, and 10.
XP was such a massive leap from 98 (2000 was more for enterprise use so it wasn't very common and ME sucked ass), and was so intuitive for its time, plus I think it has the nicest default background (Bliss) of any Windows OS.
7 was basically just Vista but not a buggy mess, so it felt extremely refined, however I do think its one of the more overrated Windows OSes because 10 is essentially better in every way at this point. Despite rocking Windows 11 on all of my machines, I still prefer 10, it just looked great for a desktop environment and feels snappier, I also greatly prefer the boxy, square design over the rounded, MacOS look of 11. That being said, certain things about 11 do look more refined, but 11 doesn't really offer enough at the moment to make it better than 10.
It absolutely bugs the crap out of me how pointlessly bigger certain things like the taskbar and UI are in 11 compared to 10, 10 looked neat and compact which was great.